Mark Lord wrote:
Tejun Heo wrote:
As the code is being smart against retrying needlessly, it won't be
too dangerous to increase the 20 tries (taken from Alan's patch) but I
think it's as good as any other random number. If anyone knows any
meaningful number, please chime in. The same goes for 60 secs timeout
too.
..
I really think that we should enforce a strict upper limit on the time
that can be spent inside the flush-cache near-infinite loop being
introduced.
Some things rely on I/O completing or failing in a time deterministic
manner.
Really, the entire flush + retries etc.. should never, ever, be permitted
to take more than XX seconds total. Not 60 seconds per retry, but XX
seconds
total for the original command + however many retries we can fit in there.
As for the value of XX, well.. make it a sysfs attribute, with a default
of something "sensible". The time bounds is really dependent upon how
quickly the drive can empty its onboard cache, or how large a cache it has.
Figure the biggest drives will have no more than, say 64MB of cache for
many years (biggest SATA drive now uses 16MB). Assuming near-worst case
I/O size of 4KB, that's 16384 I/O operations, if none were adjacent on
disk.
What's the average access time these days? Say.. 20ms worst case for any
drive with a cache that huge? That's unrealistically slow for data that's
already in the drive cache, but .. 16384 * .020 seconds = 328 seconds.
Absolute theoretical worst case for a drive with a buffer 4X the largest
current size: 328 seconds. Not taking into account having bad-sector
retries for each of those I/O blocks, but *nobody* is going to wait
that long anyway. They'll have long since pulled the power cord or
reached for the BIG RED BUTTON.
On a 16MB cache drive, that number would be 328 / 4 = 82 seconds.
That's what I'd put for the limit.
But we could be slighly nonsensical and agree upon 120 seconds. :)
Cheers
I think that the 30 seconds was meant to be that worst case time for the drive
to respond to a command. We try to push vendors to respond in much less time
than that (it's important to get things like the fast fail path for RAID working
correctly), say something like 10-15 seconds.
ric
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