From: "Martin K. Petersen" <martin.petersen@oracle.com>
To: Eric Wheeler <bcache@lists.ewheeler.net>
Cc: linux-block@vger.kernel.org, dm-devel@redhat.com,
linux-raid@vger.kernel.org, linux-kernel@vger.kernel.org,
linux-bcache@vger.kernel.org
Subject: Re: To add, or not to add, a bio REQ_ROTATIONAL flag
Date: Thu, 28 Jul 2016 21:16:16 -0400 [thread overview]
Message-ID: <yq1y44l2qhr.fsf@sermon.lab.mkp.net> (raw)
In-Reply-To: <alpine.LRH.2.11.1607281603530.10662@mail.ewheeler.net> (Eric Wheeler's message of "Thu, 28 Jul 2016 17:50:14 -0700 (PDT)")
>>>>> "Eric" == Eric Wheeler <bcache@lists.ewheeler.net> writes:
Eric,
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
next prev parent reply other threads:[~2016-07-29 1:16 UTC|newest]
Thread overview: 4+ messages / expand[flat|nested] mbox.gz Atom feed top
2016-07-29 0:50 To add, or not to add, a bio REQ_ROTATIONAL flag Eric Wheeler
2016-07-29 1:04 ` Wols Lists
2016-07-29 1:16 ` Martin K. Petersen [this message]
2016-08-01 2:58 ` Eric Wheeler
Reply instructions:
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
Avoid top-posting and favor interleaved quoting:
https://en.wikipedia.org/wiki/Posting_style#Interleaved_style
* Reply using the --to, --cc, and --in-reply-to
switches of git-send-email(1):
git send-email \
--in-reply-to=yq1y44l2qhr.fsf@sermon.lab.mkp.net \
--to=martin.petersen@oracle.com \
--cc=bcache@lists.ewheeler.net \
--cc=dm-devel@redhat.com \
--cc=linux-bcache@vger.kernel.org \
--cc=linux-block@vger.kernel.org \
--cc=linux-kernel@vger.kernel.org \
--cc=linux-raid@vger.kernel.org \
/path/to/YOUR_REPLY
https://kernel.org/pub/software/scm/git/docs/git-send-email.html
* If your mail client supports setting the In-Reply-To header
via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line
before the message body.
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for NNTP newsgroup(s).