samba - Sep 2008 - sync always, strict sync, cache question

Hello,

I've been using samba to share folders for a number of years now.
thankyou very much to the samba team.

Now I'm posed with some questions.
I have a couple of servers hosting desktops for thinclient users.
Until now I have been mounting /home with NFS on these servers.
Recently we have upgraded the whole system and have run into write
performance issues with the NFS server.

time dd if=/dev/zero of=/home/testfile bs=16k count=16384
268435456 bytes (268 MB) copied, 45.0461 seconds, 6.0 MB/s

This is much too slow for the gigabit network.
We can improve this a lot by exporting /home with the async option
(60-70MB/s), but the NFS documentation strongly recommends against
using async (opposed to sync) because of possible firesystem
corruption if the NFS server crashes. This is a cache issue.

After numerous tests we have given up trying to get it working
properly. And we've been trying CIFS.
Everything works well and write speed is a lot better.

time dd if=/dev/zero of=./testfile bs=16k count=16384
268435456 bytes (268 MB) copied, 3.6 seconds, 74.4 MB/s

But I am worried about the cache that Samba makes use of. We would
like samba to write to disk immediately.
We've found these two options for smb.conf

sync always = yes
strict sync = yes

I can't quiet see the difference between the two in my case.
If I set 'sync always = yes' _or_ 'strict sync = yes', I can
copy
files at 70MB/s (similar to NFS using async).
If I set both options, file transfer speed drops to about 20MB/s

Does that mean that I do need to set both options to ensure the cache
is written to disk before the server returns the ok to the client?
How could I test this?

And now while I'm here ;) , does anyone have any other recommendations
for this kind of setup?

Thanks,
Chris.

Chris Fanning wrote:
>snip
> 
> But I am worried about the cache that Samba makes use of. We would
> like samba to write to disk immediately.
> We've found these two options for smb.conf
> 
> sync always = yes
> strict sync = yes
> 
> I can't quiet see the difference between the two in my case.
> If I set 'sync always = yes' _or_ 'strict sync = yes', I
can copy
> files at 70MB/s (similar to NFS using async).
> If I set both options, file transfer speed drops to about 20MB/s
> 
> Does that mean that I do need to set both options to ensure the cache
> is written to disk before the server returns the ok to the client?
> How could I test this?
> 
> And now while I'm here ;) , does anyone have any other recommendations
> for this kind of setup?
> 
> Thanks,
> Chris.
Hi Chris,  I did an investigation on this in 2003.  Here are the 
results.  Not sure if things have changed since then.
---------------------------------------------------------------


Samba defaults to asynchronous writes.  smbd writes to memory buffer, 
then returns to processing.  Buffer is flushed to disk later. This is 
the most efficient behavior.

Windows CreateFile API has the FILE_FLAG_WRITE_THROUGH flag, which 
requests synchronous writes.  smbd writes to memory buffer, blocks until 
buffer contents are written to disk, which results in poor performance, 
but better data integrity.

When "strict sync = yes" (default = no) Samba honors the 
FILE_FLAG_WRITE_THROUGH flag, and results in synchronous writes when 
called by the CreateFile API.

When "sync always = yes" (default = no) Samba executes all writes 
synchronously. This requires that ?strict sync = yes?.

StrictSync  SyncAlways  ff_write_through   Sync-Writes
     no          no             no                no
     yes         no             no                no
     yes         no             yes               Yes (slow)
     no          yes            yes               no
     yes         yes            yes/no            yes (very slow)

Eric Roseme