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 | [email protected] /\/\|=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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