Filesystem snapshots dog slow

[Eric Anderson]

Kostik Belousov wrote:
> On Wed, Oct 17, 2007 at 08:00:03PM +1000, Peter Jeremy wrote:
>> 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.
> This is, unfortunately, quite true. Allowing non-atomic updates of the
> cg block means a lot of complications in the softupdate code, IMHO.


I agree with all the above.  I think it has not been done because of 
exactly what Kostik says.  I really think that the CG max size is *way* 
too small now, and should be about 10-50 times larger, but performance 
tests would need to be run.

Eric