On Wed, Oct 17, 2007 at 08:00:03PM +1000, Peter Jeremy wrote: > On 2007-Oct-16 06:54:11 -0500, Eric Anderson <[EMAIL PROTECTED]> 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.
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