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-boun...@opensolaris.org [mailto:zfs-discuss-boun...@opensolaris.org
] On Behalf Of Richard Elling
Sent: Saturday, 14 November 2009 5:02 AM
To: Rodrigo E. De León Plicet
Cc: zfs-discuss@opensolaris.org
Subject: Re: [zfs-discuss] Snapshot question
On Nov 13, 2009, at 6:43 AM, Rodrigo E. De León 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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