From: Julian Andres Klode Subject: Re: Please help: Is ext4 counting trims as writes, or is something killing my SSD? Date: Thu, 12 Sep 2013 17:32:32 +0200 Message-ID: <20130912153232.GA19548@jak-x230> References: <20130912141856.GA17640@jak-x230> <1378997643.28638.53.camel@hp-a6734f> <5231DB33.9090104@redhat.com> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Cc: Calvin Walton , linux-ext4@vger.kernel.org To: Eric Sandeen Return-path: Received: from mail-bk0-f54.google.com ([209.85.214.54]:37385 "EHLO mail-bk0-f54.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753760Ab3ILPch (ORCPT ); Thu, 12 Sep 2013 11:32:37 -0400 Received: by mail-bk0-f54.google.com with SMTP id mz12so4271809bkb.41 for ; Thu, 12 Sep 2013 08:32:35 -0700 (PDT) Content-Disposition: inline In-Reply-To: <5231DB33.9090104@redhat.com> Sender: linux-ext4-owner@vger.kernel.org List-ID: On Thu, Sep 12, 2013 at 10:18:11AM -0500, Eric Sandeen wrote: > On 9/12/13 9:54 AM, Calvin Walton wrote: > > On Thu, 2013-09-12 at 16:18 +0200, Julian Andres Klode wrote: > >> Hi, > >> > >> I installed my new laptop on Saturday and setup an ext4 filesystem > >> on my / and /home partitions. Without me doing much file transfers, > >> I noticed today: > >> > >> jak@jak-x230:~$ cat /sys/fs/ext4/sdb3/lifetime_write_kbytes > >> 342614039 > >> > >> This is on a 100GB partition. I used fstrim multiple times. I analysed > >> the increase over some time today and issued an fstrim in between: > > > >> So it seems that ext4 counts the trims as writes? I don't know how I could > >> get 300GB of writes on a 100GB partition -- of which only 8 GB are occupied > >> -- otherwise. > > > > The way fstrim works is that it allocates a temporary file that fills > > almost the entire free space on the partition. > > No, that's not correct. > > > I believe it does this > > with fallocate in order to ensure that space for the file is actually > > reserved on disc (but it does not get written to!). It then looks up > > where on disc the file's reserved space is, and sends a trim command to > > the drive to free that space. Afterwards, it deletes the temporary file. > > Nope. ;) strace it and see, it does nothing like this - it calls a special > ioctl to ask the fs to find and issue discards on unused blocks. > > # strace -e open,write,fallocate,unlink,ioctl fstrim mnt/ > open("/etc/ld.so.cache", O_RDONLY) = 3 > open("/lib64/libc.so.6", O_RDONLY) = 3 > open("/usr/lib/locale/locale-archive", O_RDONLY) = 3 > open("mnt/", O_RDONLY) = 3 > ioctl(3, 0xc0185879, 0x7fff6ac47d40) = 0 <=== FITRIM ioctl > > (old hdparm discard might have done what you say, but that was a hack). > > > So what you are seeing means means that it's probably just an issue with > > the write accounting, where the blocks reserved by the fallocate are > > counted as writes. > > I also think that it is just accounting, and probably just an error, > which seems to be fixed by now - what kernel are you running? Kernel 3.10.7 > > When you report it in ext4, it calculates it like this: > > return snprintf(buf, PAGE_SIZE, "%llu\n", > (unsigned long long)(sbi->s_kbytes_written + > ((part_stat_read(sb->s_bdev->bd_part, sectors[1]) - > EXT4_SB(sb)->s_sectors_written_start) >> 1))); > > so it counts partition stats in the mix (outside of ext4's accounting) > > On io completion, we add the bytes "completed" (blk_account_io_completion()) > > And it sounds like it's counting trim/discard completions in the mix. > > does /proc/diskstats show a jump for your partition after an fstrim as well? > I created a file using fallocate, deleted it (with discard option set on the FS), and then sync'ed and got the following changes in sdb3: jak@jak-x230:~$ diff /tmp/a /tmp/b diff --git tmp/a tmp/b index e0370bf..43c2fdd 100644 --- tmp/a +++ tmp/b @@ -1,7 +1,7 @@ 8 0 sda 1845 2122 15992 15268 6070 313375 3119314 5359680 0 85548 5391508 8 1 sda1 500 0 3970 1104 4106 37774 2840016 1028656 0 29656 1046320 - 8 16 sdb 85114 4486 4281300 36344 143239 111626 282319450 1803288 0 101416 1839608 + 8 16 sdb 85114 4486 4281300 36344 143300 111658 284417426 1803492 0 101460 1839812 8 17 sdb1 930 992 8152 316 2 0 2 0 0 68 316 8 18 sdb2 72071 3316 3024626 29692 54309 29582 23201808 183432 0 37704 213060 - 8 19 sdb3 11858 175 1246458 6320 88381 82044 259117640 1619624 0 65880 1626200 + 8 19 sdb3 11858 175 1246458 6320 88442 82076 261215616 1619828 0 65924 1626404 > > But what kernel are you running? I don't see it on a 3.11 kernel: > > After a fresh mkfs I'm at: > [root@bp-05 tmp]# dumpe2fs -h fsfile | grep Lifetime > dumpe2fs 1.41.12 (17-May-2010) > Lifetime writes: 8135 MB > > and then several fstrims don't budge it: > > [root@bp-05 tmp]# cat /sys/fs/ext4/loop0/lifetime_write_kbytes > 8330683 > [root@bp-05 tmp]# fstrim mnt/ > [root@bp-05 tmp]# cat /sys/fs/ext4/loop0/lifetime_write_kbytes > 8330683 > [root@bp-05 tmp]# fstrim mnt/ > [root@bp-05 tmp]# cat /sys/fs/ext4/loop0/lifetime_write_kbytes > 8330683 > > -Eric -- Julian Andres Klode - Debian Developer, Ubuntu Member See http://wiki.debian.org/JulianAndresKlode and http://jak-linux.org/.