Lustre devel - Nov 2008 - Lustre client disk cache (fscache)

Greetings,

We (System Fabric Works) have been retained by Sun to prove concept on
integrating the lustre client filesystem with a local disk cache
(specifically fscache from Red Hat).

Eric Barton and I discussed this several days ago, but I would appreciate
others'' feedback on the requirements and approach documented  below. 
I''m
relatively new to Lustre, so there is a very real possibility  that I
"don''t

know what I don''t know".

Motivation

This work is primarily motivated by the need to improve the performance
of lustre clients as SMB servers to windows nodes.  As I understand it,
this need is primarily for file readers.

Requirements

1. Enabling fscache should be a mount option, and there should be ioctl
   support for enabling, disabling and querying a file''s fscache usage.
2. Data read into the page cache will be asynchronously copied to the
   disk-based fscache upon arrival.
3. If requested data is not present in the page cache, it will be retrieved
   preferentially from the fscache.  If not present in the fscache, data
   will be read via RPC.
4. When pages are reclaimed due to memory pressure, they should remain in
   the fscache.
5. When a user writes a page (if we support fscache for non-read-only
opens),
   the corresponding fscache page must either be invalidated or
   (more likely) rewritten.
6. When a DLM lock is revoked, the entire extent of the lock must be
   dropped from the fscache (in addition to dropping any page cache
   resident pages) - regardless of whether any pages are currently resident
   in the page cache.
7. As sort-of a corollary to #6, DLM locks must not be canceled by the owner
   as long as pages are resident in the fscache, even if memory pressure
   reclamation has emptied the page cache for a given file.
8. Utilities and test programs will be needed, of course.
9. The fscache must be cleared upon mount or dismount.

High Level Design Points

The following is written based primarily on review of the 1.6.5.1 code.
I''m aware that this is not the place for new development, but it was
deemed a stable place for initial experimentation.

Req.    Notes

 1.    In current Redhat distributions, fscache is included and
    NFS includes fscache support, enabled by a mount option.
    We don''t see any problems with doing something similar.
    A per-file ioctl to enable/disable fscache usage is also seen
    as straightforward.

 2.     When an RPC read (into the page cache) completes, in the
    ll_ap_completion() function, an asynchronous read to the
    same offset in the file''s fscache object will be initiated.
    This should not materially impact access time (think dirty page
    to fscache filesystem).

 3.     When the readpage method is invoked because a page is not
    already resident in the page cache, the page will be read
    first from the fscache.  This is non-blocking and (presumably)
    fast for the non-resident case.  If available, the fscache
    read will proceed asynchronously, after which the page will be
    valid in the page cache.  If not available in the fscache,
    the RPC read will proceed normally.

 4.     Page removal due to memory pressure is triggered by a call to
    the llap_shrink_cache function.  This function should not require
    any material change, since pages can be removed from the page
    cache without removal from the fscache in this case.  In fact,
    if this doesn''t happen, the fscache will never be read.
    (note: test coverage will be important here)

 5.    It may be reasonable in early code to enable fscache only
    for read-only opens.  However, we don''t see any inherent problems
    with running an asynchronous write to the fscache concurrently
    with a Lustre RPC write.  Note that this approach would *never*
    have dirty pages exist only in the fscache; if it''s dirty it
    stays in the page cache until it''s written via RPC (or RPC
    AND fscache if we''re writing to both places)..

 6 & 7    This is where it gets a little more tedious.  Let me revert to
    paragraph form to address these cases below.

 8    Testing will require the following:
    * ability to query and punch holes in the page cache (already done).
    * ability to query and punch holes in the fscache (nearly done).

 9  I presume that all locks are canceled when a client dismounts
    a filesystem, in which case it would never be safe to use data
    in the fscache from a prior mount.


Lock Revocation

Please apply that "it looks to me like this is how things work" filter
here;
I am still pretty new to Lustre (thanks).  My questions are summarized
after the the text of this section.

As of 1.6.5.1, DLM locks keep a list of page-cached pages
(lock->l_extents_list contains osc_async_page structs for all currently
cached pages - and I think the word extent is used both for each page cached
under a lock, and to describe a locked region...is this right?).  If a lock
is revoked, that list is torn down and the pages are freed.  Pages are also
removed from that list when they are freed due to memory pressure, making
that list sparse with regard to the actual region of the lock.

Adding fscache, there will be zero or more page-cache pages in the extent
list, as well as zero or more pages in the file object in the fscache.
The primary question, then, is whether a lock will remain valid (i.e. not be
voluntarily released) if all of the page-cache pages are freed for
non-lock-related reasons (see question 3 below).

The way I foresee cleaning up the fscache is by looking at the overall
extent of the lock (at release or revocation time), and punching a
lock-extent-sized hole in the fscache object prior to looping through
the page list (possibly in cache_remove_lock() prior to calling
cache_remove_extents_from_lock()).

However, that would require finding the inode, which (AFAICS) is not
available in that context (ironically, unless the l_extents_list is non-
empty, in which case the inode can be found via any of the page structs in
the list).  I have put in a hack to solve this, but see question 6 below.

Summarized questions:
Q1: Where can I read up on the unit testing infrastructure for Lustre?
Q2: Is stale cache already covered by existing unit tests?
Q3: Will a DLM lock remain valid (i.e. not be canceled) even if its page
    list is empty (i.e. all pages have been freed due to memory pressure)?
Q4: Will there *always* be a call to cache_remove_lock() when a lock is
    canceled or revoked?  (i.e. is this the place to punch a hole in the
    fscache object?)
Q5: for the purpose of punching a hole in a cache object upon lock
    revocation, can I rely on the lock->l_req_extent structure as the
    actual extent of the lock?
Q6: a) is there a way to find the inode that I''ve missed?, and
    b) if not what is the preferred way of giving that function a way to
    find the inode?

...

FYI we have done some experimenting and we have the read path in a
demonstrable state, including crude code to effect lock revocation on the
fscache contents.  The NFS code modularized the fscache hooks pretty nicely,
and we have followed that example.

Thanks,
John Groves
John at SystemFabricWorks.com
+1-512-302-4005
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John,

Unfortunately I can''t help you much with the coding but I just thought
I''d drop a quick email to let you know that we are extremely interested
in this feature. We have used large memory cache NFS export servers to speed up
small file metadata workloads already and we are looking to use the same system
to provide WLAN access. The idea being that our London renderfarm is closely
coupled to the storage but yet our remote (New York and Iceland) offices can
work interactively with the resulting data. The disk cache will obviously
greatly increase the cache retention. We are more than willing to test this code
when it gets to a working state - good luck!

Daire


----- "John Groves" <jgl at johngroves.net> wrote:
> Greetings,
> 
> We (System Fabric Works) have been retained by Sun to prove concept on
> integrating the lustre client filesystem with a local disk cache
> (specifically fscache from Red Hat).
> 
> Eric Barton and I discussed this several days ago, but I would
> appreciate
> others'' feedback on the requirements and approach documented
below.
> I''m
> relatively new to Lustre, so there is a very real possibility that I
> "don''t
> know what I don''t know".
> 
> Motivation
> 
> This work is primarily motivated by the need to improve the
> performance
> of lustre clients as SMB servers to windows nodes. As I understand it,
> this need is primarily for file readers.
> 
> Requirements
> 
> 1. Enabling fscache should be a mount option, and there should be
> ioctl
> support for enabling, disabling and querying a file''s fscache
usage.
> 2. Data read into the page cache will be asynchronously copied to the
> disk-based fscache upon arrival.
> 3. If requested data is not present in the page cache, it will be
> retrieved
> preferentially from the fscache. If not present in the fscache, data
> will be read via RPC.
> 4. When pages are reclaimed due to memory pressure, they should remain
> in
> the fscache.
> 5. When a user writes a page (if we support fscache for non-read-only
> opens),
> the corresponding fscache page must either be invalidated or
> (more likely) rewritten.
> 6. When a DLM lock is revoked, the entire extent of the lock must be
> dropped from the fscache (in addition to dropping any page cache
> resident pages) - regardless of whether any pages are currently
> resident
> in the page cache.
> 7. As sort-of a corollary to #6, DLM locks must not be canceled by the
> owner
> as long as pages are resident in the fscache, even if memory pressure
> reclamation has emptied the page cache for a given file.
> 8. Utilities and test programs will be needed, of course.
> 9. The fscache must be cleared upon mount or dismount.
> 
> High Level Design Points
> 
> The following is written based primarily on review of the 1.6.5.1
> code.
> I''m aware that this is not the place for new development, but it
was
> deemed a stable place for initial experimentation.
> 
> Req. Notes
> 
> 1. In current Redhat distributions, fscache is included and
> NFS includes fscache support, enabled by a mount option.
> We don''t see any problems with doing something similar.
> A per-file ioctl to enable/disable fscache usage is also seen
> as straightforward.
> 
> 2. When an RPC read (into the page cache) completes, in the
> ll_ap_completion() function, an asynchronous read to the
> same offset in the file''s fscache object will be initiated.
> This should not materially impact access time (think dirty page
> to fscache filesystem).
> 
> 3. When the readpage method is invoked because a page is not
> already resident in the page cache, the page will be read
> first from the fscache. This is non-blocking and (presumably)
> fast for the non-resident case. If available, the fscache
> read will proceed asynchronously, after which the page will be
> valid in the page cache. If not available in the fscache,
> the RPC read will proceed normally.
> 
> 4. Page removal due to memory pressure is triggered by a call to
> the llap_shrink_cache function. This function should not require
> any material change, since pages can be removed from the page
> cache without removal from the fscache in this case. In fact,
> if this doesn''t happen, the fscache will never be read.
> (note: test coverage will be important here)
> 
> 5. It may be reasonable in early code to enable fscache only
> for read-only opens. However, we don''t see any inherent problems
> with running an asynchronous write to the fscache concurrently
> with a Lustre RPC write. Note that this approach would *never*
> have dirty pages exist only in the fscache; if it''s dirty it
> stays in the page cache until it''s written via RPC (or RPC
> AND fscache if we''re writing to both places)..
> 
> 6 & 7 This is where it gets a little more tedious. Let me revert to
> paragraph form to address these cases below.
> 
> 8 Testing will require the following:
> * ability to query and punch holes in the page cache (already done).
> * ability to query and punch holes in the fscache (nearly done).
> 
> 9 I presume that all locks are canceled when a client dismounts
> a filesystem, in which case it would never be safe to use data
> in the fscache from a prior mount.
> 
> 
> Lock Revocation
> 
> Please apply that "it looks to me like this is how things work"
filter
> here;
> I am still pretty new to Lustre (thanks). My questions are summarized
> after the the text of this section.
> 
> As of 1.6.5.1 , DLM locks keep a list of page-cached pages
> (lock->l_extents_list contains osc_async_page structs for all
> currently
> cached pages - and I think the word extent is used both for each page
> cached
> under a lock, and to describe a locked region...is this right?). If a
> lock
> is revoked, that list is torn down and the pages are freed. Pages are
> also
> removed from that list when they are freed due to memory pressure,
> making
> that list sparse with regard to the actual region of the lock.
> 
> Adding fscache, there will be zero or more page-cache pages in the
> extent
> list, as well as zero or more pages in the file object in the fscache.
> The primary question, then, is whether a lock will remain valid (i.e.
> not be
> voluntarily released) if all of the page-cache pages are freed for
> non-lock-related reasons (see question 3 below).
> 
> The way I foresee cleaning up the fscache is by looking at the overall
> extent of the lock (at release or revocation time), and punching a
> lock-extent-sized hole in the fscache object prior to looping through
> the page list (possibly in cache_remove_lock() prior to calling
> cache_remove_extents_from_lock()).
> 
> However, that would require finding the inode, which (AFAICS) is not
> available in that context (ironically, unless the l_extents_list is
> non-
> empty, in which case the inode can be found via any of the page
> structs in
> the list). I have put in a hack to solve this, but see question 6
> below.
> 
> Summarized questions:
> Q1: Where can I read up on the unit testing infrastructure for Lustre?
> Q2: Is stale cache already covered by existing unit tests?
> Q3: Will a DLM lock remain valid (i.e. not be canceled) even if its
> page
> list is empty (i.e. all pages have been freed due to memory pressure)?
> Q4: Will there *always* be a call to cache_remove_lock() when a lock
> is
> canceled or revoked? (i.e. is this the place to punch a hole in the
> fscache object?)
> Q5: for the purpose of punching a hole in a cache object upon lock
> revocation, can I rely on the lock->l_req_extent structure as the
> actual extent of the lock?
> Q6: a) is there a way to find the inode that I''ve missed?, and
> b) if not what is the preferred way of giving that function a way to
> find the inode?
> 
> ...
> 
> FYI we have done some experimenting and we have the read path in a
> demonstrable state, including crude code to effect lock revocation on
> the
> fscache contents. The NFS code modularized the fscache hooks pretty
> nicely,
> and we have followed that example.
> 
> Thanks,
> John Groves
> John at SystemFabricWorks.com
> +1-512-302-4005 
> _______________________________________________
> Lustre-devel mailing list
> Lustre-devel at lists.lustre.org
> http://lists.lustre.org/mailman/listinfo/lustre-devel

On Nov 11, 2008  13:23 -0600, John Groves wrote:
> This work is primarily motivated by the need to improve the performance
> of lustre clients as SMB servers to windows nodes.  As I understand it,
> this need is primarily for file readers.
> 
> Requirements
> 
> 1. Enabling fscache should be a mount option, and there should be ioctl
>    support for enabling, disabling and querying a file''s fscache
usage.
For Lustre there should also be the ability to do this via /proc/fs/lustre
tunables/stats.
> 2. Data read into the page cache will be asynchronously copied to the
>    disk-based fscache upon arrival.
> 3. If requested data is not present in the page cache, it will be retrieved
>    preferentially from the fscache.  If not present in the fscache, data
>    will be read via RPC.
> 4. When pages are reclaimed due to memory pressure, they should remain in
>    the fscache.
> 5. When a user writes a page (if we support fscache for non-read-only
> opens),
>    the corresponding fscache page must either be invalidated or
>    (more likely) rewritten.
> 6. When a DLM lock is revoked, the entire extent of the lock must be
>    dropped from the fscache (in addition to dropping any page cache
>    resident pages) - regardless of whether any pages are currently resident
>    in the page cache.
> 7. As sort-of a corollary to #6, DLM locks must not be canceled by the
owner
>    as long as pages are resident in the fscache, even if memory pressure
>    reclamation has emptied the page cache for a given file.
> 8. Utilities and test programs will be needed, of course.
> 9. The fscache must be cleared upon mount or dismount.
> High Level Design Points
> 
> The following is written based primarily on review of the 1.6.5.1 code.
> I''m aware that this is not the place for new development, but it
was
> deemed a stable place for initial experimentation.
Note that the client IO code was substantially re-written for the 2.0
release.  The client IO code from 1.6.5 is still present through the
1.8.x releases.
> Req.    Notes
> 
>  1.    In current Redhat distributions, fscache is included and
>     NFS includes fscache support, enabled by a mount option.
>     We don''t see any problems with doing something similar.
>     A per-file ioctl to enable/disable fscache usage is also seen
>     as straightforward.
> 
>  2.     When an RPC read (into the page cache) completes, in the
>     ll_ap_completion() function, an asynchronous read to the
>     same offset in the file''s fscache object will be initiated.
>     This should not materially impact access time (think dirty page
>     to fscache filesystem).
Do you mean an "asynchronous write to the ... fscache object"?
>  3.     When the readpage method is invoked because a page is not
>     already resident in the page cache, the page will be read
>     first from the fscache.  This is non-blocking and (presumably)
>     fast for the non-resident case.  If available, the fscache
>     read will proceed asynchronously, after which the page will be
>     valid in the page cache.  If not available in the fscache,
>     the RPC read will proceed normally.
> 
>  4.     Page removal due to memory pressure is triggered by a call to
>     the llap_shrink_cache function.  This function should not require
>     any material change, since pages can be removed from the page
>     cache without removal from the fscache in this case.  In fact,
>     if this doesn''t happen, the fscache will never be read.
>     (note: test coverage will be important here)
> 
>  5.    It may be reasonable in early code to enable fscache only
>     for read-only opens.  However, we don''t see any inherent
problems
>     with running an asynchronous write to the fscache concurrently
>     with a Lustre RPC write.  Note that this approach would *never*
>     have dirty pages exist only in the fscache; if it''s dirty it
>     stays in the page cache until it''s written via RPC (or RPC
>     AND fscache if we''re writing to both places)..
This is dangerous from the point of view that the write to the fscache
may succeed, but the RPC may fail for a number of reasons (e.g. client
eviction) so it would seem that the write to the fscache cannot start
until the RPC completes successfully.
>  6 & 7    This is where it gets a little more tedious.  Let me revert
to
>     paragraph form to address these cases below.
> 
>  8    Testing will require the following:
>     * ability to query and punch holes in the page cache (already done).
>     * ability to query and punch holes in the fscache (nearly done).
> 
>  9  I presume that all locks are canceled when a client dismounts
>     a filesystem, in which case it would never be safe to use data
>     in the fscache from a prior mount.
A potential future improvement in the second generation of this feature
might be the ability to revalidate the files in the local disk cache by
the MDT and OST object versions, if those are also stored in fscache.
> Lock Revocation
> 
> Please apply that "it looks to me like this is how things work"
filter here;
> I am still pretty new to Lustre (thanks).  My questions are summarized
> after the the text of this section.
> 
> As of 1.6.5.1, DLM locks keep a list of page-cached pages
> (lock->l_extents_list contains osc_async_page structs for all currently
> cached pages - and I think the word extent is used both for each page
cached
> under a lock, and to describe a locked region...is this right?).  If a lock
> is revoked, that list is torn down and the pages are freed.  Pages are also
> removed from that list when they are freed due to memory pressure, making
> that list sparse with regard to the actual region of the lock.
> 
> Adding fscache, there will be zero or more page-cache pages in the extent
> list, as well as zero or more pages in the file object in the fscache.
> The primary question, then, is whether a lock will remain valid (i.e. not
be
> voluntarily released) if all of the page-cache pages are freed for
> non-lock-related reasons (see question 3 below).
Yes, the lock can remain valid on the client even when no pages are
protected by the lock.  However, locks with few pages are more likely
to be cancelled by the DLM LRU because the cost of re-fetching those
locks is much smaller compared to locks covering lots of data.  The
lock "weight" function would need to be enhanced to include pages that
are in fscache instead of just those in memory.
> The way I foresee cleaning up the fscache is by looking at the overall
> extent of the lock (at release or revocation time), and punching a
> lock-extent-sized hole in the fscache object prior to looping through
> the page list (possibly in cache_remove_lock() prior to calling
> cache_remove_extents_from_lock()).
> 
> However, that would require finding the inode, which (AFAICS) is not
> available in that context (ironically, unless the l_extents_list is non-
> empty, in which case the inode can be found via any of the page structs in
> the list).  I have put in a hack to solve this, but see question 6 below.
Actually, each lock has a back-pointer to the inode that is referencing
it, in l_ast_data, so that lock_cancel->mapping->page_removal can work.
Use ll_inode_from_lock() for this.
> Summarized questions:
> Q1: Where can I read up on the unit testing infrastructure for Lustre?
There is an internal wiki page with some information on this, it should
probably be moved to the public wiki.
> Q2: Is stale cache already covered by existing unit tests?
I''m not sure what you mean.  There is no such thing as stale cache in
Lustre.
> Q3: Will a DLM lock remain valid (i.e. not be canceled) even if its page
>     list is empty (i.e. all pages have been freed due to memory pressure)?
Yes, though the reverse is impossible.
> Q4: Will there *always* be a call to cache_remove_lock() when a lock is
>     canceled or revoked?  (i.e. is this the place to punch a hole in the
>     fscache object?)
> Q5: for the purpose of punching a hole in a cache object upon lock
>     revocation, can I rely on the lock->l_req_extent structure as the
>     actual extent of the lock?
No, there are two different extent ranges on each lock.  The requested
extent, and the granted extent.  The requested extent is the minimum
extent size that the server could possibly grant to the client to finish
the operation (e.g. large enough to handle a single read or write syscall).
The server may decide to grant a larger lock if the resource (object) is
not contended.

In the current implementation, the DLM will always grant a full-file lock
to the first client that requests it, because the most common application
case is that only a single client is accessing the file.  This avoids any
future lock requests for this file in the majority of cases.
> Q6: a) is there a way to find the inode that I''ve missed?, and
>     b) if not what is the preferred way of giving that function a way to
>     find the inode?
See above.
> FYI we have done some experimenting and we have the read path in a
> demonstrable state, including crude code to effect lock revocation on the
> fscache contents.  The NFS code modularized the fscache hooks pretty
nicely,
> and we have followed that example.
Cheers, Andreas
--
Andreas Dilger
Sr. Staff Engineer, Lustre Group
Sun Microsystems of Canada, Inc.

Andreas, thanks for the thoughtful reply, and sorry for being so slow to
acknowledge and respond to it.   Responses are below.

On Fri, Nov 14, 2008 at 6:00 PM, Andreas Dilger <adilger at sun.com>
wrote:
> On Nov 11, 2008  13:23 -0600, John Groves wrote:
> > This work is primarily motivated by the need to improve the
performance
> > of lustre clients as SMB servers to windows nodes.  As I understand
it,
> > this need is primarily for file readers.
> >
> > Requirements
> >
> > 1. Enabling fscache should be a mount option, and there should be
ioctl
> >    support for enabling, disabling and querying a file''s
fscache usage.
>
> For Lustre there should also be the ability to do this via /proc/fs/lustre
> tunables/stats.

Makes sense, thanks.

>
>
> > 2. Data read into the page cache will be asynchronously copied to the
> >    disk-based fscache upon arrival.
> > 3. If requested data is not present in the page cache, it will be
> retrieved
> >    preferentially from the fscache.  If not present in the fscache,
data
> >    will be read via RPC.
> > 4. When pages are reclaimed due to memory pressure, they should remain
in
> >    the fscache.
> > 5. When a user writes a page (if we support fscache for non-read-only
> > opens),
> >    the corresponding fscache page must either be invalidated or
> >    (more likely) rewritten.
> > 6. When a DLM lock is revoked, the entire extent of the lock must be
> >    dropped from the fscache (in addition to dropping any page cache
> >    resident pages) - regardless of whether any pages are currently
> resident
> >    in the page cache.
> > 7. As sort-of a corollary to #6, DLM locks must not be canceled by the
> owner
> >    as long as pages are resident in the fscache, even if memory
pressure
> >    reclamation has emptied the page cache for a given file.
> > 8. Utilities and test programs will be needed, of course.
> > 9. The fscache must be cleared upon mount or dismount.
>
> > High Level Design Points
> >
> > The following is written based primarily on review of the 1.6.5.1
code.
> > I''m aware that this is not the place for new development, but
it was
> > deemed a stable place for initial experimentation.
>
> Note that the client IO code was substantially re-written for the 2.0
> release.  The client IO code from 1.6.5 is still present through the
> 1.8.x releases.

Understood.

> > Req.    Notes
> >
> >  1.    In current Redhat distributions, fscache is included and
> >     NFS includes fscache support, enabled by a mount option.
> >     We don''t see any problems with doing something similar.
> >     A per-file ioctl to enable/disable fscache usage is also seen
> >     as straightforward.
> >
> >  2.     When an RPC read (into the page cache) completes, in the
> >     ll_ap_completion() function, an asynchronous read to the
> >     same offset in the file''s fscache object will be
initiated.
> >     This should not materially impact access time (think dirty page
> >     to fscache filesystem).
>
>
> Do you mean an "asynchronous write to the ... fscache object"?

Yes - write it is.

> >  3.     When the readpage method is invoked because a page is not
> >     already resident in the page cache, the page will be read
> >     first from the fscache.  This is non-blocking and (presumably)
> >     fast for the non-resident case.  If available, the fscache
> >     read will proceed asynchronously, after which the page will be
> >     valid in the page cache.  If not available in the fscache,
> >     the RPC read will proceed normally.
> >
> >  4.     Page removal due to memory pressure is triggered by a call to
> >     the llap_shrink_cache function.  This function should not require
> >     any material change, since pages can be removed from the page
> >     cache without removal from the fscache in this case.  In fact,
> >     if this doesn''t happen, the fscache will never be read.
> >     (note: test coverage will be important here)
> >
> >  5.    It may be reasonable in early code to enable fscache only
> >     for read-only opens.  However, we don''t see any inherent
problems
> >     with running an asynchronous write to the fscache concurrently
> >     with a Lustre RPC write.  Note that this approach would *never*
> >     have dirty pages exist only in the fscache; if it''s dirty
it
> >     stays in the page cache until it''s written via RPC (or
RPC
> >     AND fscache if we''re writing to both places)..
>
>
> This is dangerous from the point of view that the write to the fscache
> may succeed, but the RPC may fail for a number of reasons (e.g. client
> eviction) so it would seem that the write to the fscache cannot start
> until the RPC completes successfully.

Good catch, thanks.

> >  6 & 7    This is where it gets a little more tedious.  Let me
revert to
> >     paragraph form to address these cases below.
> >
> >  8    Testing will require the following:
> >     * ability to query and punch holes in the page cache (already
done).
> >     * ability to query and punch holes in the fscache (nearly done).
> >
> >  9  I presume that all locks are canceled when a client dismounts
> >     a filesystem, in which case it would never be safe to use data
> >     in the fscache from a prior mount.
>
>
> A potential future improvement in the second generation of this feature
> might be the ability to revalidate the files in the local disk cache by
> the MDT and OST object versions, if those are also stored in fscache.

Cool idea.

> > Lock Revocation
> >
> > Please apply that "it looks to me like this is how things
work" filter
> here;
> > I am still pretty new to Lustre (thanks).  My questions are summarized
> > after the the text of this section.
> >
> > As of 1.6.5.1, DLM locks keep a list of page-cached pages
> > (lock->l_extents_list contains osc_async_page structs for all
currently
> > cached pages - and I think the word extent is used both for each page
> cached
> > under a lock, and to describe a locked region...is this right?).  If a
> lock
> > is revoked, that list is torn down and the pages are freed.  Pages are
> also
> > removed from that list when they are freed due to memory pressure,
making
> > that list sparse with regard to the actual region of the lock.
> >
> > Adding fscache, there will be zero or more page-cache pages in the
extent
> > list, as well as zero or more pages in the file object in the fscache.
> > The primary question, then, is whether a lock will remain valid (i.e.
not
> be
> > voluntarily released) if all of the page-cache pages are freed for
> > non-lock-related reasons (see question 3 below).
>
>
> Yes, the lock can remain valid on the client even when no pages are
> protected by the lock.  However, locks with few pages are more likely
> to be cancelled by the DLM LRU because the cost of re-fetching those
> locks is much smaller compared to locks covering lots of data.  The
> lock "weight" function would need to be enhanced to include pages
that
> are in fscache instead of just those in memory.

Got it, thanks.  That would have eluded me...

> > The way I foresee cleaning up the fscache is by looking at the overall
> > extent of the lock (at release or revocation time), and punching a
> > lock-extent-sized hole in the fscache object prior to looping through
> > the page list (possibly in cache_remove_lock() prior to calling
> > cache_remove_extents_from_lock()).
>
FYI it turns out that fscache doesn''t have the ability to punch a hole.
The
whole file has to be dropped at present.

>
> >
> > However, that would require finding the inode, which (AFAICS) is not
> > available in that context (ironically, unless the l_extents_list is
non-
> > empty, in which case the inode can be found via any of the page
structs
> in
> > the list).  I have put in a hack to solve this, but see question 6
below.
>
>
> Actually, each lock has a back-pointer to the inode that is referencing
> it, in l_ast_data, so that lock_cancel->mapping->page_removal can
work.
> Use ll_inode_from_lock() for this.

That''s much nicer than my hack...thanks.

> > Summarized questions:
> > Q1: Where can I read up on the unit testing infrastructure for Lustre?
>
>
> There is an internal wiki page with some information on this, it should
> probably be moved to the public wiki.

If there''s a way to let me know when that happens, I''d
appreciate it.  I''m
not a full time lustre-devel reader (at least currently).


> > Q2: Is stale cache already covered by existing unit tests?
>
>
> I''m not sure what you mean.  There is no such thing as stale cache
in
> Lustre.

What I was driving at is a test to verify that any page cache data was
discarded when a lock was revoked.  The same test would catch failure to
discard fscache data, that being a potentially stale place to reload the
page cache from.  Perhaps that''s implicitly covered somehow.

> > Q3: Will a DLM lock remain valid (i.e. not be canceled) even if its
page
> >     list is empty (i.e. all pages have been freed due to memory
> pressure)?
>
>
> Yes, though the reverse is impossible.
>
> > Q4: Will there *always* be a call to cache_remove_lock() when a lock
is
> >     canceled or revoked?  (i.e. is this the place to punch a hole in
the
> >     fscache object?)
> > Q5: for the purpose of punching a hole in a cache object upon lock
> >     revocation, can I rely on the lock->l_req_extent structure as
the
> >     actual extent of the lock?
>
> No, there are two different extent ranges on each lock.  The requested
> extent, and the granted extent.  The requested extent is the minimum
> extent size that the server could possibly grant to the client to finish
> the operation (e.g. large enough to handle a single read or write syscall).
> The server may decide to grant a larger lock if the resource (object) is
> not contended.
>
> In the current implementation, the DLM will always grant a full-file lock
> to the first client that requests it, because the most common application
> case is that only a single client is accessing the file.  This avoids any
> future lock requests for this file in the majority of cases.

Thanks.  Given that fscache invalidation turns out to be full-file anyway,
this becomes moot for the time being.

> > Q6: a) is there a way to find the inode that I''ve missed?,
and
> >     b) if not what is the preferred way of giving that function a way
to
> >     find the inode?
>
>
> See above.
>
> > FYI we have done some experimenting and we have the read path in a
> > demonstrable state, including crude code to effect lock revocation on
the
> > fscache contents.  The NFS code modularized the fscache hooks pretty
> nicely,
> > and we have followed that example.
>
> Cheers, Andreas
> --
> Andreas Dilger
> Sr. Staff Engineer, Lustre Group
> Sun Microsystems of Canada, Inc.
>
>
>
Thanks again!
John
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