[mythtv-users] Single WD GP disk fast enough for live 720p?
jon at whitear.org
Wed Dec 2 05:45:13 UTC 2009
> > Please excuse my ignorance here (relatively new to RAID etc.) but
> what's not
> > to support?
> > Surely the OS just sees the drive as a lump of blocks and allocates
> > accordingly?
> > Also, I have two of the 1TB in a mirrored software RAID and they are
> > great. Wonder what went wrong with the 1.5TB versions.
> The problem is if the heads are automatically parked and the raid is
> called on the software raid may assume the drive is bad if it takes
> too long to recover from the parked heads and kick the drive out of
> the array.
With the WD drives, it seems there are a couple of issues that have caused problems for RAID users. One is the head-park count(aka load cycle) rapidly increasing - IIRC the drives have a rated life of 300,000 load cycles, and people were seeing it increase at a rate of many hundreds per day, leading to an expected life of <3 years. Not much good, but not actually a performance issue.
The other problem was error recovery - basically the drive takes too long trying to recover an error and the host OS RAID software concludes that it's dead, so drops it from the array. This has been the real killer for all the NAS users.
There are (non-WD-supported) utilities available to increase the head parking delay time (WDIDLE) and disable the error recovery (WDTLER) but neither of them worked on my firmware version of the WD15EADS. I don't know whether the performance issue I had is related to either of the problems above. However, moving my software RAID1s from my existing 300 & 320 GB Seagate PATA drives to the WD15EADS saw iowait hit 100% on one of four cores, and load increase from <0.5 to >5. The system became unusable.
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