Sync option destroys flash!

From: Michael H. Warfield (mhw_at_wittsend.com)
Date: 05/13/05

  • Next message: Greg KH: "Re: [PATCH] Updated: fix-pci-mmap-on-ppc-and-ppc64.patch" 
    To: linux-kernel@vger.kernel.org
Date:	Fri, 13 May 2005 12:20:06 -0400

            Under the right circumstances, even copying a single file to a flash
    drive mounted with the "sync" option can destroy the entire drive!

            Now that I have your attention!

            I found this out the hard way. (Kissed one brand new $70 USD 1GB flash
    drive good-bye.) According to the man pages for mount, FAT and VFAT
    file systems ignore the "sync" option. It lies. Maybe it use to be
    true, but it certainly lies now. A simple test can verify this. Mount
    a flash drive with a FAT/VFAT file system without the sync option and
    writes to the drive go very fast. Typing "sync" or unmounting the drive
    afterwards, takes time as the buffered data is written to the flash
    drive. This is as it should be. Mount it with the sync option and
    writes are really REALLY slow (worse than they should be just from
    copying the data through USB) but sync and umount come back immediately
    and result in no additional writing to the drive. [Do the preceding
    with only a few files and less than a few meg of data if you value that
    flash.] So... FAT and VFAT are honoring the sync option. This is very
    VERY bad. It's bad for floppies, it's bad for hard drives, it's FATAL
    for flash drives.

            Flash drives have a limited number of write cycles. Many many
    thousands of write cycles, but limited, none the less. They are also
    written in blocks which are much larger than the "sector" size report
    (several K in a physical nand flash block, IRC).

            What happens, with the sync option on a VFAT file system, is that the
    FAT tables are getting pounded and over-written over and over and over
    again as each and every block/cluster is allocated while a new file is
    written out. This constant overwriting eventually wears out the first
    block or two of the flash drive.

            I had lost a couple of flash keys previously, without realizing what
    was going on, but what send me off investigating this was when I copied
    a 700 Meg file to a brand new 1G USB 2.0 flash memory key in a USB 2.0
    slot. It took over a half an hour to copy to the drive, which really
    had me wondering WTF was wrong! Then, when I went to use the key, I
    found the first couple of blocks were totally destroyed. Read errors
    immediately upon insertion. Then I started digging and found the
    hotplug / HAL / fstab-sync stuff on Fedora Core was mounting USB drives
    with the "sync" option (if less than 2 Gig). I knew from previous
    experience (CF backup cards in a PDA) that repeated pounding on the FAT
    tables would destroy a flash card with a FAT file system. So I reported
    this on the Fedora list. Someone else noticed that the man pages for
    mount state that FAT and VFAT ignore the sync option. Not so,
    obviously... Copying that 700 Meg file resulted in thousands upon
    thousands upon thousands of writes to the FAT table and backup FAT
    table. It simply blew through all the rewrites for those blocks and
    burned them up. Bye bye flash key...

            On a floppy, this would result in an insane amount of jacking around
    back and forth between data sectors and the FAT sectors. In addition to
    taking forever, that would shorten the life of the diskettes and the
    drive itself, but who cares about floppies any more. On a real hard
    drive, this will cause "head resonances" as the heads go through
    constant high speed seeks between the cylinder with the FAT tables and
    the data cylinders. That can't be good, on a continuous basis, for
    drive life. But it's really a disaster for flash memory. It's going to
    cause premature failure in most flash memory, even if it doesn't kill
    them right off as it did in my case with a 700 Meg file.

            Can we go back to ignoring "sync" on FAT and VFAT? I can't see where
    it does much good. You might corrupt a file system if you unplugged it
    while dirty but it beats the hell out of physically burning it up and
    destroying the drive!

            If it's decided that the FAT and VFAT file systems MUST obey the sync
    option then please do something about a special case for the FAT tables!
    Sync the data if thou must buti... Thou shalt not, must not, whack off
    on the FAT tables!!!

            Another option would be to only sync the FAT and VFAT file systems upon
    close of the file being written or upon close of the last file open on
    the file system (fs not busy) but that might not help in the case of a
    whole lotta little files...

            I'm also going to file a couple of bug reports in bugzilla at RedHat
    but this seems to be a more fundamental problem than a RedHat specific
    problem. But, IMHO, they should never be setting that damn sync flag
    arbitrarily.

            Mike 

    -- 
 Michael H. Warfield    |  (770) 985-6132   |  mhw@WittsEnd.com  
  /\/\|=mhw=|\/\/       |  (678) 463-0932   |  http://www.wittsend.com/mhw/
  NIC whois:  MHW9      |  An optimist believes we live in the best of all
 PGP Key: 0xDF1DD471    |  possible worlds.  A pessimist is sure of it!

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  • Next message: Greg KH: "Re: [PATCH] Updated: fix-pci-mmap-on-ppc-and-ppc64.patch"