As of linux-2.6.36 the BIO_RW_FAILFAST and REQ_FAILFAST flags
have been unified under the REQ_* names. These flags always had
to be kept in-sync so this is a nice step forward, unfortunately
it means we need to be careful to only use the new unified flags
when the BIO_RW_* flags are not defined. Additional autoconf
checks were added for this and if it is ever unclear which method
to use no flags are set. This is safe but may result in longer
delays before a disk is failed.
Perferred interface for setting FAILFAST on a bio:
2.6.12-2.6.27: BIO_RW_FAILFAST
2.6.28-2.6.35: BIO_RW_FAILFAST_{DEV|TRANSPORT|DRIVER}
2.6.36-2.6.xx: REQ_FAILFAST_{DEV|TRANSPORT|DRIVER}
ZFS works best when it is notified as soon as possible when a device
failure occurs. This allows it to immediately start any recovery
actions which may be needed. In theory Linux supports a flag which
can be set on bio's called FAILFAST which provides this quick
notification by disabling the retry logic in the lower scsi layers.
That's the theory at least. In practice is turns out that while the
flag exists you oddly have to set it with the BIO_RW_AHEAD flag.
And even when it's set it you may get retries in the low level
drivers decides that's the right behavior, or if you don't get the
right error codes reported to the scsi midlayer.
Unfortunately, without additional kernels patchs there's not much
which can be done to improve this. Basically, this just means that
it may take 2-3 minutes before a ZFS is notified properly that a
device has failed. This can be improved and I suspect I'll be
submitting patches upstream to handle this.
