On Fri, Jan 15, 2010 at 08:41:55AM -0800, Linus Torvalds wrote:
> [root@EeePC torvalds]# time blkid -o udev -p -u filesystem /dev/sdc1
> ID_FS_SEC_TYPE=msdos
> ID_FS_VERSION=FAT12
> ID_FS_TYPE=vfat
> ID_FS_USAGE=filesystem
>
> real 0m9.233s
> user 0m0.003s
> sys 0m0.003s
>
> because it still does that large read:
>
> read(3, "\353<\220){'`jIHC\0\2\10\1\0\2\0\2\333\3\370\1\0?\0\377\0 \0\0\0"..., 69632) = 69632
>
Yes, because it always reads 69kB. I'll try to fix it.
> (but then it does _not_ do the 15+ small reads at the end).
>
> It still does two other reads:
>
> _llseek(3, 270336, [270336], SEEK_SET) = 0
> read(3, "\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0"..., 1024) = 1024
this is zfs probing
> _llseek(3, 262144, [262144], SEEK_SET) = 0
> read(3, "\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0"..., 1377) = 1377
this is ufs probing
> I dunno what those are about. It's at about the middle of the disk. Might
> be some vfat tables.
The libblkid allows to restrict probing by "usage" (raid, filesystem,
crypt, ...) or by filesystem name. The restriction by fs name is not
exported to blkid(8) command line now. I my TODO is something like
blkid -o udev -p -n fat,ext2,ext3 /dev/sdc1
then it will probe for requested filesystems only.
> > or -u noraid
>
> Same thing. "-u noraid" seems to do exactly the same reads and seeks as
> "-u filesystem".
>
> > the other way is to use -S <size> to restrict probing
> > area. (Unfortunately, it seems that -S doesn't work now, I'll fix it
> > asap.)
>
> Well, I'd have to restrict it to the first few kB of the device to get rid
> of that insane 70kB read.
>
> You can definitely find a FAT12 filesystem in way less than 70kB.
For example Reiser superblok could be at offset 65536.
The other problem is that we always (by default) probe for all
filesystems, because sometimes people have disks with more valid
signatures -- we have to ignore such disks, because mount/swapon
could be pretty dangerous.
(http://karelzak.blogspot.com/2009_11_01_archive.html)
Anyway, I agree that it does not make sense to probe for things like
GFS, OCFS, RAIDs, ... on really small devices.
I'll fix -S and remove the large 69kB read and then you can create a
special udev rule for the device. I'm not sure if we need to change
globally the way who udev uses blkid(8). It's first time when someone
complains ;-)
(except the stupid situation when we call blkid(8) for whole
partitioned disk)
Karel
--
Karel Zak <kzak@xxxxxxxxxx>
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