On 24/10/19 23:50, Dave Chinner wrote:
On Wed, Oct 23, 2019 at 11:40:33AM +0200, Gionatan Danti wrote: Defaults are for best compatibility and general behaviour, not best performance. A log stripe unit of 32kB allows the user to configure a logbsize appropriate for their workload, as it supports logbsize of 32kB, 64kB, 128kB and 256kB. If we chose 256kB as the default log stripe unit, then you have no opportunity to set the logbsize appropriately for your workload. remember, LSU determines how much padding is added to every non-full log write - 32kB pads out ot 32kB, 256kB pads out to 256kB. Hence if you have a workload that frequnetly writes non-full iclogs (e.g. regular fsyncs) then a small LSU results in much better performance as there is less padding that needs to be initialised and the IOs are much smaller. Hence for the general case (i.e. what the defaults are aimed at), a small LSU is a much better choice. you can still use a large logbsize mount option and it will perform identically to a large LSU filesystem on full iclog workloads (like the above fsmark workload that doesn't use fsync). However, a small LSU is likely to perform better over a wider range of workloads and storage than a large LSU, and so small LSU is a better choice for the default....
Hi Dave, thank you for your explanation. The observed behavior of a large LSU surely matches what you described - less-than-optimal fsync perf.
That said, I was wondering why *logbsize* (rather than LSU) has a low default of 32k (or, better, its default is to match LSU size). If I understand it correctly, a large logbsize (eg: 256k) on top of a small LSU (32k) would give high performance on both full-log-writes and partial-log-writes (eg: frequent fsync).
Is my understanding correct? If you, do you suggest to always set logbsize to the maximum supported value?
Thanks. -- Danti Gionatan Supporto Tecnico Assyoma S.r.l. - www.assyoma.it email: g.danti@xxxxxxxxxx - info@xxxxxxxxxx GPG public key ID: FF5F32A8
