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:
> > <snip>
> >> 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/.
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