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Re: [dm-devel] [PATCHSET block#for-2.6.36-post] block: replace barrier with sequenced flush



Hello,

Tejun Heo, on 08/13/2010 05:21 PM wrote:
If requested, I can develop the interface further.

I still think the benefit of ordering by tag would be marginal at
best, and what have you guys measured there?  Under the current
framework, there's no easy way to measure full ordered-by-tag
implementation.  The mechanism for filesystems to communicate the
ordering information (which would be a partially ordered graph) just
isn't there and there is no way the current usage of ordering-by-tag
only for barrier sequence can achieve anything close to that level of
difference.

Basically, I measured how iSCSI link utilization depends from amount of queued commands and queued data size. This is why I made it as a table. From it you can see which improvement you will have removing queue draining after 1, 2, 4, etc. commands depending of commands sizes.

For instance, on my previous XFS rm example, where rm of 4 files took 3.5 minutes with nobarrier option, I could see that XFS was sending 1-3 32K commands in a row. From my table you can see that if it sent all them at once without draining, it would have about 150-200% speed increase.

Vlad


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