On Tue, 22 May 2012 01:36:06 -0500 Stan Hoeppner <stan@xxxxxxxxxxxxxxxxx> wrote: > On 5/21/2012 6:34 PM, NeilBrown wrote: > > On Mon, 21 May 2012 13:51:21 -0500 Stan Hoeppner <stan@xxxxxxxxxxxxxxxxx> > > wrote: > > > >> On 5/21/2012 10:20 AM, CoolCold wrote: > >>> On Sat, May 12, 2012 at 2:28 AM, Stan Hoeppner <stan@xxxxxxxxxxxxxxxxx> wrote: > >>>> On 5/11/2012 3:16 AM, Daniel Pocock wrote: > >>>> > >>> [snip] > >>>> That's the one scenario where I abhor using md raid, as I mentioned. At > >>>> least, a boot raid 1 pair. Using layered md raid 1 + 0, or 1 + linear > >>>> is a great solution for many workloads. Ask me why I say raid 1 + 0 > >>>> instead of raid 10. > >>> So, I'm asking - why? > >> > >> Neil pointed out quite some time ago that the md RAID 1/5/6/10 code runs > >> as a single kernel thread. Thus when running heavy IO workloads across > >> many rust disks or a few SSDs, the md thread becomes CPU bound, as it > >> can only execute on a single core, just as with any other single thread. > > > > This is not the complete truth. > > Yes, I should have stipulated only writes are limited to a single thread. > > > For RAID1 and RAID10, successful IO requests do not involved the kernel > > thread, so the fact that there is only one should be irrelevant. > > Failed requests are retried using the thread and it is also involved it > > resync/recovery so those processes may be limited by the single thread. > > > > RAID5/6 does not use the thread for read requests on a non-degraded array. > > However all write requests go through the single thread so there could be > > issues there. > > Thanks for clarifying this. In your previous response to this issue > (quoted and linked below) you included RAID 1/10 with RAID 5/6 WRT > writes going through a single thread. > > > Have you actually measured md/raid10 being slower than raid0 over raid1? > > I personally have not, as I don't have access to the storage hardware > necessary to sink a sufficiently large write stream to peak a core with > the md thread. > > > I have a vague memory from when this came up before that there was some extra > > issue that I was missing, but I cannot recall it just now.... > > We're recalling the same thread, which was many months ago. Here's your > post: http://marc.info/?l=linux-raid&m=132616899005148&w=2 > > And here's the relevant section upon which I was basing my recent > statements: > > "I think you must be misremembering. Neither RAID0 or Linear have any > threads involved. They just redirect the request to the appropriate > devices. Multiple threads can submit multiple requests down through > RAID0 and Linear concurrently. > > RAID1, RAID10, and RAID5/6 are different. For reads they normally are > have no contention with other requests, but for writes things do get > single-threaded at some point." > That's right - I keep forgetting about the single-threading caused by needing to sync with bitmap updates. So reads a fully parallel. Writes are - for now - serialised to a single processor for handling. Thanks, NeilBrown
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