linux-ext4 - Re: [PATCH 8/9] e2scrub_all: refactor device probe loop

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Date:   Thu, 21 Mar 2019 22:14:37 +0100
From:   Lukas Czerner <lczerner@...hat.com>
To:     Theodore Ts'o <tytso@....edu>
Cc:     Ext4 Developers List <linux-ext4@...r.kernel.org>,
        darrick.wong@...cle.com
Subject: Re: [PATCH 8/9] e2scrub_all: refactor device probe loop

On Thu, Mar 21, 2019 at 04:48:23PM -0400, Theodore Ts'o wrote:
> On Thu, Mar 21, 2019 at 09:17:10PM +0100, Lukas Czerner wrote:
> > 
> > Right, I did mention it later in the reply. It can be filtered
> > 
> > grep -v '^/dev/'
> 
> Well, that assumes all device nodes are in /dev.  Which is not
> necessarily always the case, especially in some of the more _whacky_
> container setups which I've seen.   (Hmm, is whacky redundant here?)

Fair enough, I've never seen this outside dm/lvm testing.

> 
> I suppose can test whether or not the path is a block device or a
> directory.....
> 
> > 
> > For me this new function is the wors of all.
> > 
> > cold cache:
> > real	0m2.115s
> > user	0m0.040s
> > sys	0m0.154s
> > 
> > second time:
> > real	0m1.100s
> > user	0m0.037s
> > sys	0m0.122s
> > 
> > But that's because of blkid which is terribly slow for some reason.
> 
> I ran my test on a system with a NVMe SSD, and no HDD's attached.  I
> just did an strace, and I see the util-linux folks have really done a
> great job of pessimizing blkid.  :-(

Yeah, all I have on that system is spinning rust :)
lsblk works good enough for me so I am not sure how I feel about special
binary to check the mount point :)

-Lukas

> 
> My version used to just pull the information from the blkid cache file
> and then verified the results, but it looks like the new, improved
> util-linux version of blkid scans the /dev directory and opens and
> reads from each device node, even when we're querying a single block
> device.  <groan>
> 
> Even lsblk is *amazingly* inefficient in terms of the number of
> useless file opens which it performs, although at least they are all
> /sysfs files.
> 
> I'm half tempted to create and ship a binary which just calls
> ext2fs_check_mount_point() and returns the value, since it's the most
> efficient.  This command
> 
> sudo strace -o /tmp/st /build/e2fsprogs-maint/lib/ext2fs/tst_ismounted  /dev/lambda/tp
> 
> Opens /proc/mounts and does a trial mount of /dev/lambda/tp to make
> sure it's actually busy, and that's it.
> 
> Sigh...
> 
> 						- Ted