[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index] Re: [Xen-users] File-based domU - Slow storage write since xen 4.8
On 07/21/17 18:07, Roger Pau Monné wrote: > On Fri, Jul 21, 2017 at 11:53:33AM -0400, Keith Busch wrote: >> On Fri, Jul 21, 2017 at 12:19:39PM +0200, Benoit Depail wrote: >>> On 07/20/17 19:36, Keith Busch wrote: >>>> >>>> As a test, could you throttle the xvdb queue's max_sectors_kb? If I >>>> followed xen-blkfront correctly, the default should have it set to 44. >>>> Try setting it to 40. >>>> >>>> echo 40 > /sys/block/xvdb/queue/max_sectors_kb >>>> >>> >>> The default value on my domU is 128. >>> >>> I ran a couple of tests with different values, starting from 40 and up >>> to 128, clearing the cache between each tests. >>> >>> The only value that showed the issue is 128. Even setting max_sectors_kb >>> to 127 is enough to get normal behaviour. >> >> Ok, I don't quite follow how it's initialized to 128k, but your >> results appear to confirm the default settings are not optimal for the >> interface. The patch you identified just submits commands to the max >> size the driver asked to use. If the largest size tanks performance, >> I think xen-blkfront should register a smaller transfer size, or maybe >> some other constraint needs to be refined. >> >> Roger, >> >> I'm a bit out of my depth here in the xen code. Is this something you >> may be able to help clarify? > > Hm, I'm not sure I follow either. AFAIK this problem came from > changing the Linux version in the Dom0 (where the backend, blkback is > running), rather than in the DomU right? > > Regarding the queue/sectors stuff, blkfront uses several blk_queue > functions to set those parameters, maybe there's something wrong > there, but I cannot really spot what it is: > > http://elixir.free-electrons.com/linux/latest/source/drivers/block/xen-blkfront.c#L929 > > In the past the number of pages that could fit in a single ring > request was limited to 11, but some time ago indirect descriptors > where introduced in order to lift this limit, and now requests can > have a much bigger number of pages. > > Could you check the max_sectors_kb of the underlying storage you are > using in Dom0? > > Roger. > I checked the value for the loop device as well With 4.4.77 (bad write performance) $ cat /sys/block/sda/queue/max_sectors_kb 1280 $ cat /sys/block/loop1/queue/max_sectors_kb 127 With 4.1.42 (normal write performance) $ cat /sys/block/sda/queue/max_sectors_kb 4096 $ cat /sys/block/loop1/queue/max_sectors_kb 127 Regards, -- Benoit Depail Senior Infrastructures Architect NBS System _______________________________________________ Xen-users mailing list Xen-users@xxxxxxxxxxxxx https://lists.xen.org/xen-users
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