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> wrote: >> >> > Ping to Neil / the raid list. >> >> Thanks for the reminder. >> >> > [snip] > >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. > >Cheers, > >Dave. >-- >Dave Chinner >david@xxxxxxxxxxxxx >-- >To unsubscribe from this list: send the line "unsubscribe linux-raid" in >the body of a message to majordomo@xxxxxxxxxxxxxxx >More majordomo info at http://vger.kernel.org/majordomo-info.html?韬{.n?????%??檩??w?{.n???{炳盯w???塄}?财??j:+v?? ? ???2??璀??摺?囤??z夸z罐?+?????w棹f
