Re: Feature Request

["Stefan Hübner" <stefan.huebner@stud.tu-ilmenau.de>]

Am 09.02.2010 13:28, schrieb Michael Tokarev:
> Stefan *St0fF* Huebner wrote:
> []
>> Now imagine any RAID with some kind of redundancy, reading/writing
>> data.  One of the disks finds out "I cannot correctly read/write the
>> requested sector", starts its error correction, hits the respective
>> ERC-timeout and reports back a media error or unrecoverable error.  Now
>> mdraid would drop the disk.
>>
>> But actually the data of the sector can be recreated through the
>> existing redundancy.  Wouldn't it be a smart thing if the mdraid
>> recreates the sector and just tried to write it again?  And after a good
>> amount of failed retries it may well drop the disk.
>
> This is exactly what md layer is doing.  On failed _read_ it tries to
> reconstruct data from other disk drives and writes the reconstructed
> data back to the drive where read failed.  If the _write_ fails md will
> drop the disk.
Hi Mjt,

I hoped so - great it is implemented like that.

Well, then all that's needed is the check at assembly/creation time:
- (is the drive an ATA-drive) && (does it support SCT ERC)
-> and if it does, set some reasonable timeouts. (like the 7s it is with
enterprise class drives for reading.  For writing I would suggest 14s,
bearing in mind that too quick reallocation results in the spare sectors
running out quickly.)

The writing back (I guess this is done with a reasonable amount of
retries) does not make sense if the drive is still in its error recovery
procedure and does not react to any commands until it is done.

P.S.: I have already implemented the checks and setup, but in userspace
using SG_IO.

/st0ff
>
> /mjt
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