Filesystem snapshots dog slow

[Peter Jeremy]

On 2007-Oct-16 06:54:11 -0500, Eric Anderson <anderson at freebsd.org> wrote:
>will give you a good understanding of what the issue is. Essentially, your 
>disk is hammered making copies of all the cylinder groups, skipping those 
>that are 'busy', and coming back to them later. On a 200Gb disk, you could 
>have 1000 cylinder groups, each having to be locked, copied, unlocked, and 
>then checked again for any subsequent changes.  The stalls you see are when 
>there are lock contentions, or disk IO issues.  On a single disk (like your 
>setup above), your snapshots will take forever since there is very little 
>random IO performance available to you.

That said, there is a fair amount of scope available for improving
both the creation and deletion performance.

Firstly, it's not clear to me that having more than a few hundred CGs
has any real benefits.  There was a massive gain in moving from
(effectively) a single CG in pre-FFS to a few dozen CGs in FFS as it
was first introduced.  Modern disks are roughly 5 orders of magnitude
larger and voice-coil actuators mean that seek times are almost
independent of distance.  CG sizes are currently limited by the
requirement that the cylinder group (including cylinder group maps)
must fit into a single FS block.  Removing this restriction would
allow CGs to be much larger.

Secondly, all the I/O during both snapshot creation and deletion is
in FS-block size chunks.  Increasing the I/O size would significantly
increase the I/O performance.  Whilst it doesn't make sense to read
more than you need, there still appears to be plenty of scope to
combine writes.

Between these two items, I would expect potential performance gains
of at least 20:1.

Note that I'm not suggesting that either of these items is trivial.

-- 
Peter Jeremy
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