On 2012-07-02 16:08 Dave Chinner <david@xxxxxxxxxxxxx> Wrote:
>On Mon, Jul 02, 2012 at 04:41:13PM +1000, NeilBrown wrote:
>> On Mon, 2 Jul 2012 02:18:27 -0400 Christoph Hellwig <hch@xxxxxxxxxxxxx>
>> > Ping to Neil / the raid list.
>> Thanks for the reminder.
>That's true, but the characterisitics of spinning disks have not
>changed in the past 20 years, nor has the typical file size
>distributions in filesystems, nor have the RAID5/6 algorithms. So
>it's not really clear to me why you;d woul deven consider changing
>the default the downsides of large chunk sizes on RAID5/6 volumes is
>well known. This may well explain the apparent increase in "XFS has
>hung but it's really just waiting for lots of really slow IO on MD"
>cases I've seen over the past couple of years.
At present, cat /sys/block/sdb/queue/max_sectors_kb:
is 512k. Maybe because this.
>The only time I'd ever consider stripe -widths- of more than 512k or
>1MB with RAID5/6 is if I knew my workload is almost exclusively
>using large files and sequential access with little metadata load,
>and there's relatively few workloads where that is the case.
>Typically those workloads measure throughput in GB/s and everyone
>uses hardware RAID for them because MD simply doesn't scale to this
>sort of usage.
>> If 512K is always suboptimal for XFS then that is unfortunate but I don't
>I think 512k chunk sizes are suboptimal for most users, regardless
>of the filesystem or workload....
>> think it is really possible to choose a default that everyone will be happy
>> with. Maybe we just need more documentation and warning emitted by various
>> tools. Maybe mkfs.xfs could augment the "stripe unit too large" message with
>> some text about choosing a smaller chunk size?
>We work to the mantra that XFS should always choose the defaults
>that give the best overall performance and aging characteristics so
>users don't need to be a storage expert to get the best the
>filesystem can offer. The XFS warning is there to indicate that the
>user might be doing something wrong. If that's being emitted with a
>default MD configuration, then that indicates that the MD defaults
>need to be revised....
>If you know what a stripe unit or chunk size is, then you know how
>to deal with the problem. But for the majority of people, that's way
>more knowledge than they are prepared to learn about or should be
>forced to learn about.
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