zfs discuss - Nov 2009 - Snapshot question

While reading about NILFS here:

http://www.linux-mag.com/cache/7345/1.html


I saw this:

*One of the most noticeable features of NILFS is that it can
"continuously
> and automatically save instantaneous states of the file system without
> interrupting service". NILFS refers to these as checkpoints. In
contrast,
> other file systems such as ZFS, can provide snapshots but they have to
suspend
> operation to perform the snapshot operation. NILFS doesn?t have to do
> this. The snapshots (checkpoints) are part of the file system design
itself.
> *
>
I don''t think that''s correct. Can someone clarify?
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>While reading about NILFS here:
>
>http://www.linux-mag.com/cache/7345/1.html
>
>
>I saw this:
>
>*One of the most noticeable features of NILFS is that it can
"continu>ously
>> and automatically save instantaneous states of the file system
with>out
>> interrupting service". NILFS refers to these as checkpoints. In
con>trast,
>> other file systems such as ZFS, can provide snapshots but they have>
to suspend
>> operation to perform the snapshot operation. NILFS doesn=E2=80=99t
>have to do
>> this. The snapshots (checkpoints) are part of the file system
desig>n itself.
>> *
>>
>
>I don''t think that''s correct. Can someone clarify?
No, and they also write this:
>More over, creating these checkpoints or snapshots do not result
>in decreased performance as they do for file systems such as ZFS.
Both are wrong; a snapshot is cheap and doesn''t suspend operations.
It''s pretty much like the checkpoints in NILFS.

Casper

On Nov 13, 2009, at 6:43 AM, Rodrigo E. De Le?n Plicet wrote:
> While reading about NILFS here:
>
> http://www.linux-mag.com/cache/7345/1.html
>
>
> I saw this:
>
> One of the most noticeable features of NILFS is that it can  
> "continuously and automatically save instantaneous states of the  
> file system without interrupting service". NILFS refers to these as  
> checkpoints. In contrast, other file systems such as ZFS, can  
> provide snapshots but they have to suspend operation to perform the  
> snapshot operation. NILFS doesn?t have to do this. The snapshots  
> (checkpoints) are part of the file system design itself.
>
> I don''t think that''s correct. Can someone clarify?
It sounds to me like they confused Solaris UFS with ZFS.  What they
say applies to UFS, but not ZFS.
  -- richard

I think the exception may be when doing a recursive snapshot - ZFS appears to
halt IO so that it can take all the snapshots at the same instant.

At least, that''s what it looked like to me. I''ve got an
Opensolaris ZFS box providing NFS to VMWare, and I was getting SCSI
timeout''s within the Virtual Machines that appeared to happen exactly
as the snapshots were taken.

When I turned off the recursive snapshots, and rather had each FS snapshot
individually, the problem went away.

Regards,
	Tristan.

-----Original Message-----
From: zfs-discuss-bounces at opensolaris.org [mailto:zfs-discuss-bounces at
opensolaris.org] On Behalf Of Richard Elling
Sent: Saturday, 14 November 2009 5:02 AM
To: Rodrigo E. De Le?n Plicet
Cc: zfs-discuss at opensolaris.org
Subject: Re: [zfs-discuss] Snapshot question


On Nov 13, 2009, at 6:43 AM, Rodrigo E. De Le?n Plicet wrote:
> While reading about NILFS here:
>
> http://www.linux-mag.com/cache/7345/1.html
>
>
> I saw this:
>
> One of the most noticeable features of NILFS is that it can  
> "continuously and automatically save instantaneous states of the  
> file system without interrupting service". NILFS refers to these as  
> checkpoints. In contrast, other file systems such as ZFS, can  
> provide snapshots but they have to suspend operation to perform the  
> snapshot operation. NILFS doesn''t have to do this. The snapshots  
> (checkpoints) are part of the file system design itself.
>
> I don''t think that''s correct. Can someone clarify?
It sounds to me like they confused Solaris UFS with ZFS.  What they
say applies to UFS, but not ZFS.
  -- richard

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On Nov 13, 2009, at 10:36 PM, Tristan Ball wrote:
> I think the exception may be when doing a recursive snapshot - ZFS  
> appears to halt IO so that it can take all the snapshots at the same  
> instant.
Snapshots cause a txg commit, similar to what you get when you run sync.
The time required to commit depends on many factors, perhaps the largest
of which is the latency of the disk.
>
> At least, that''s what it looked like to me. I''ve got an
Opensolaris
> ZFS box providing NFS to VMWare, and I was getting SCSI timeout''s
> within the Virtual Machines that appeared to happen exactly as the  
> snapshots were taken.
SCSI timeouts?!? How short are their timeouts? By default in Solaris,  
SCSI
timeouts are 60 seconds.  Have you seen a recursive snapshot take more
than 60 seconds?
>
> When I turned off the recursive snapshots, and rather had each FS  
> snapshot individually, the problem went away.
There have been performance tweeks over the past few years which can
impact snapshot performance, though it is still largely gated by the  
disk.
What release were you running?
  -- richard
>
> Regards,
> 	Tristan.
>
> -----Original Message-----
> From: zfs-discuss-bounces at opensolaris.org [mailto:zfs-discuss-bounces at
opensolaris.org
> ] On Behalf Of Richard Elling
> Sent: Saturday, 14 November 2009 5:02 AM
> To: Rodrigo E. De Le?n Plicet
> Cc: zfs-discuss at opensolaris.org
> Subject: Re: [zfs-discuss] Snapshot question
>
>
> On Nov 13, 2009, at 6:43 AM, Rodrigo E. De Le?n Plicet wrote:
>
>> While reading about NILFS here:
>>
>> http://www.linux-mag.com/cache/7345/1.html
>>
>>
>> I saw this:
>>
>> One of the most noticeable features of NILFS is that it can
>> "continuously and automatically save instantaneous states of the
>> file system without interrupting service". NILFS refers to these
as
>> checkpoints. In contrast, other file systems such as ZFS, can
>> provide snapshots but they have to suspend operation to perform the
>> snapshot operation. NILFS doesn''t have to do this. The
snapshots
>> (checkpoints) are part of the file system design itself.
>>
>> I don''t think that''s correct. Can someone clarify?
>
> It sounds to me like they confused Solaris UFS with ZFS.  What they
> say applies to UFS, but not ZFS.
>  -- richard
>
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> zfs-discuss at opensolaris.org
> http://mail.opensolaris.org/mailman/listinfo/zfs-discuss
>
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