Re: To add, or not to add, a bio REQ_ROTATIONAL flag

From: Martin K. Petersen
Date: Thu Jul 28 2016 - 21:16:46 EST

>>>>> "Eric" == Eric Wheeler <bcache@xxxxxxxxxxxxxxxxxx> writes:


Eric> However, just because FADV_SEQUENTIAL is flagged doesn't mean the
Eric> cache should bypass. Filesystems can fragment, and while the file
Eric> being read may be read sequentially, the blocks on which it
Eric> resides may not be. Same thing for higher-level block devices
Eric> such as dm-thinp where one might sequentially read a thin volume
Eric> but its _tdata might not be in linear order. This may imply that
Eric> we need a new way to flag cache bypass from userspace that is
Eric> neither io-priority nor fadvise driven.

Why conflate the two? Something being a background task is orthogonal to
whether it is being read sequentially or not.

Eric> So what are our options? What might be the best way to do this?

For the SCSI I/O hints I use the idle I/O priority to classify
backups. Works fine.

Eric> Are FADV_NOREUSE/FADV_DONTNEED reasonable candidates?

FADV_DONTNEED was intended for this. There have been patches posted in
the past that tied the loop between the fadvise flags and the bio. I
would like to see those revived.

Eric> Perhaps ionice could be used used, but the concept of "priority"
Eric> doesn't exactly encompass the concept of cache-bypass---so is
Eric> something else needed?

The idle class explicitly does not have a priority.

Martin K. Petersen Oracle Linux Engineering