[mythtv-commits] Ticket #10991: RFE: make recording raid friendly

[MythTV]

#10991: RFE: make recording raid friendly
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     Reporter:  dlbulk-mythtv@…       |      Owner:  danielk
         Type:  Bug Report - General  |     Status:  new
     Priority:  minor                 |  Milestone:  unknown
    Component:  MythTV - Recording    |    Version:  0.25-fixes
     Severity:  medium                |   Keywords:
Ticket locked:  0                     |
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 When recordings are written raid devices, it is often reporting that write
 is too slow and eventually the recording stops after truncating the file.

 Benchmarks on the raid device show that the throughput is far higher than
 mythtv reports.

 According to iostat output, when mythtv is recording, there is also lots
 of read activity on all of the raid disks. It is happening even if there
 are no read requests on the md device itself. This implies that there is
 partial stripe writes happening. In this case, raid has to do a read in
 order to completes the partial stripe write.

 Two factors can trigger partial stripe writes:
   1. write offset is not stripe aligned
   2. io size is not a multiple of a stripe

 Although there is no direct control of raid layout at application layer,
 it helps if mythtv performs aligned writes only and uses an io size that's
 a multiple of a raid stripe. The alignment should also be a multiple of
 the raid stripe.

 Yes, an underperforming disk can slow down writes. But partial stripe
 writes make it even slower :)

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Ticket URL: <http://code.mythtv.org/trac/ticket/10991>
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