From: Ivan Kozik <ivan@ludios.org>
To: cygwin@cygwin.com
Subject: Re: Wrong file position after writing 65537 bytes to block device
Date: Tue, 19 Dec 2017 18:18:00 -0000 [thread overview]
Message-ID: <CAKyntiLJTjk+5Qs0jAjxAUPJTzfY4R57GRaQjZLwBBeuMM++0Q@mail.gmail.com> (raw)
In-Reply-To: <9f29d418-10af-1dc7-2636-89cae1eb16f4@redhat.com>
On Tue, Dec 19, 2017 at 4:13 PM, Eric Blake <eblake@redhat.com> wrote:
> Can block devices report an unaligned offset to lseek()? If not, then when
> writing an unaligned value to a block device, don't we have to do a
> read-modify-write of the larger aligned cluster, and then put lseek() back
> to the unaligned boundary, and have extra magic in ftell() to track that we
> are at an unaligned position within the block device? But that sounds like
> a lot of nasty overhead; and that it would be better to make sure that block
> devices can report unaligned lseek() locations (caveat: I haven't tested
> what Linux does in that regards).
From what I observe on Linux, it supports writing at any offset to the
block device because it does a read-modify-write behind the scenes,
with accompanying nasty overhead (e.g. writes going at 64MB/s instead
of an "expected" 180MB/s).
I think you can observe this behavior on Linux by piping this
program's stdout to a block device (note: must be python3, not
python2):
#!/usr/bin/python3
import sys
block = b" " * 4096
while True:
sys.stdout.buffer.write(block)
sys.stdout.buffer.write(b" ")
and watching the block device activity with `dstat -d -D sdN` - you
should see a lot of reads.
Ivan
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next prev parent reply other threads:[~2017-12-19 16:36 UTC|newest]
Thread overview: 10+ messages / expand[flat|nested] mbox.gz Atom feed top
2017-12-16 6:13 Ivan Kozik
2017-12-18 16:32 ` Corinna Vinschen
2017-12-19 5:03 ` Steven Penny
2017-12-19 10:20 ` Corinna Vinschen
2017-12-19 16:36 ` Ivan Kozik
2017-12-19 17:43 ` Eric Blake
2017-12-19 18:18 ` Ivan Kozik [this message]
2017-12-19 18:36 ` Corinna Vinschen
2017-12-20 5:47 ` Ivan Kozik
2017-12-20 23:59 ` Kaz Kylheku
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