Re: agcount 33 by default for a single HDD?

[Carlos Maiolino <cmaiolino@redhat.com>]

On Tue, Jul 31, 2018 at 11:04:05PM -0600, Chris Murphy wrote:
> On Tue, Jul 31, 2018 at 10:41 PM, Eric Sandeen <sandeen@sandeen.net> wrote:
> >
> >
> > On 7/31/18 11:38 PM, Chris Murphy wrote:
> >> On Tue, Jul 31, 2018 at 10:12 PM, Dave Chinner <david@fromorbit.com> wrote:
> >>> On Tue, Jul 31, 2018 at 07:32:50PM -0600, Chris Murphy wrote:
> >>>> This seems suboptimal.
> >>>
> >>> It's actually a very useful optimisation to make on thin devices.
> >>>
> >>>> Basically this is a 750G thin volume. I don't
> >>>> have a plain partition on a device handy to try this out but I'm
> >>>> pretty certain the default is 4 AG's in that case, so I'm confused why
> >>>> by default 33 AGs are created on a thin volume. The LVM volume group
> >>>> is on a dmcrypt PV.
> >>>
> >>> It's a thin volume, therefore it advertises an optimal IO size and
> >>> alignment setting (i.e. the thin volume allocation chunk size).
> >>> Hence mkfs.xfs treats it as a "multi-disk device" and sets up
> >>> alignment and AG count appropriately.
> >>>
> >>> This is actually the right optimisation to make for sparse devices -
> >>> more AGs increase filesystem concurrency but we normally restrict it
> >>> on single spindles because each AG adds more seeks into typical
> >>> workloads and slows them down. However, the thin volume already adds
> >>> that penalty to the storage stack for us because they don't have a
> >>> linear LBA-to-physical location characteristic.  Hence we can
> >>> increase filesystem concurrency without addition performance
> >>> penalties being incurred.
> >>
> >> OK so why 33 AG's with xfsprogs 4.15, but 4 AG's with xfsprogs 4.17,
> >> when directed to the same thin LV? And also the difference in sunit
> >> and swidth?
> >>
> >
> > Did you build 4.17 with --disable-blkid?
> 
> Yep, that's what doc/INSTALL recommends. Removing that and rebuilding,
> sure enough 33 AGs, sunit 128, swidth 1024, and sectsz 4096. I do get
> a build time warning.

Where did you see that --disable-blkid is recommended? Are you using MacOS to
build it?

> 
> Building mkfs
>     [CC]     proto.o
>     [CC]     xfs_mkfs.o
> In function ‘finish_superblock_setup’,
>     inlined from ‘main’ at xfs_mkfs.c:3937:2:
> xfs_mkfs.c:3188:3: warning: ‘strncpy’ specified bound 12 equals
> destination size [-Wstringop-truncation]
>    strncpy(sbp->sb_fname, cfg->label, sizeof(sbp->sb_fname));
>    ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
>     [LD]     mkfs.xfs
> 
> 
> 
> -- 
> Chris Murphy
> --
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-- 
Carlos