MD/RAID time out writing superblock

[Andrei Tanas <andrei@tanas.ca>]

Hello,

This is about the same problem that I wrote two days ago (md gets an error
while writing superblock and fails a hard drive).

I've tried to figure out what's really going on, and as far as I can tell,
the disk doesn't really fail (as confirmed by multiple tests), it times out
trying to execute ATA_CMD_FLUSH_EXT ("at2.00 cmd ea..." in the log)
command. The reason for this I believe is that md_super_write queues the
write comand with BIO_RW_SYNCIO flag.
As I wrote before, with 32MB cache it is conceivable that it will take the
drive longer than 30 seconds (defined by SD_TIMEOUT in scsi/sd.h) to flush
its buffers.

Changing safe_mode_delay to more conservative 2 seconds should definitely
help, but is it really necessary to write the superblock synchronously when
array changes status from active to active-idle?

[90307.328266] ata2.00: exception Emask 0x0 SAct 0x0 SErr 0x0 action 0x6
frozen
[90307.328275] ata2.00: cmd ea/00:00:00:00:00/00:00:00:00:00/a0 tag 0
[90307.328277]          res 40/00:01:01:4f:c2/00:00:00:00:00/00 Emask 0x4
(timeout)
[90307.328280] ata2.00: status: { DRDY }
[90307.328288] ata2: hard resetting link
[90313.218511] ata2: link is slow to respond, please be patient (ready=0)
[90317.377711] ata2: SRST failed (errno=-16)
[90317.377720] ata2: hard resetting link
[90318.251720] ata2: SATA link up 3.0 Gbps (SStatus 123 SControl 300)
[90318.338026] ata2.00: configured for UDMA/133
[90318.338062] ata2: EH complete
[90318.370625] end_request: I/O error, dev sdb, sector 1953519935
[90318.370632] md: super_written gets error=-5, uptodate=0