On 29/10/2012 05:29, Roman Mamedov wrote:
On Sun, 28 Oct 2012 20:09:06 -0400
Miles Fidelman <mfidelman@xxxxxxxxxxxxxxxx> wrote:
Two separate issues.
The comments about "dropping out of raid" had to do with drives that are
slow to come out of sleep mode - causing hiccups when the RAID
hardware/software simply doesn't see the drive, and drops it.
There are no drives in good working order that would come out of sleep mode SO
slowly, that the Linux kernel ATA subsystem would even give up trying and
return an I/O error from it (and it's only after that point, when this begins
to become mdraid's concern).
I have yet to see even any first sign of "SATA frozen" due to drive sleep
mode, let alone to imagine this last through all the port resets and speed
step-downs the SATA driver will attempt.
So the "sleep" issue is not relevant with Linux software RAID, and if you're
still concerned that it might be, you can just reconfigure your drives so they
don't enter that sleep mode.
The same applies to the long retry times of "desktop" drives - Linux
software raid has no problem with them. Some (perhaps "many" or "all" -
I don't have the experience with hardware raid cards to say) hardware
raid cards see long read retries as a timeout on the disk, and will drop
the whole disk from the array.
Linux md raid will wait for the data to come in, and use it if it is
valid. If the disk returns an error, the md layer will re-create the
data from the other disks, then re-write the bad block. The disk will
then re-locate the bad block to one of its spare blocks, and everything
should be fine. (If the write also fails, the drive gets kicked out.)
So with software raid, there are no problems using desktop drives of any
sort in your array (assuming, of course, you don't have physical issues
such as heat generation, vibration, support contracts, etc., that might
otherwise make you prefer "raid" or "enterprise" drives).
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