ztrata dat na UFS + FreeBSD 5.1

[Roman Neuhauser]

# dan at obluda.cz / 2003-12-13 20:08:12 +0100:
> Petr Palacký wrote:
> 
> >Ten vypadek mel samozrejme za nasledek tvrdy restart, ale i tak jsem mel 
> >za to, ze UFS je celkem odolne (diky tomu jsem prave volil *BSD a ne Linux 
> >a jeho e2fs/e3fs/reiserfs) 

> 	Hlavni rozdil mezi temito systemy, tykajici se odolnosti proti 
> 	"nahlym vypadkum" pak nespociva v zakladech tehto filesystemu, ale v tom, 
> ze FreeBSD pouziva defaultne synchroni rezim (nebo soft-updates) a k 
> asynchronimu se uchylouje jen na vyslovne prani. ext2fs, jehoz vykon je 
> v synchronnim rezimu neprijatelny, pouziva defaultne rezim asynchronni a 
> jedine na zadost pouziva synchronni.
> 
> 	Nahodne vypadku jsou smrtelne nebezpecne pro asynchroni rezimy.

    nejen pro ne. tuning(7):

    IDE WRITE CACHING
    FreeBSD 4.3 flirted with turning off IDE write caching.  This reduced
    write bandwidth to IDE disks but was considered necessary due to serious
    data consistency issues introduced by hard drive vendors.	Basically the
    problem is that IDE drives lie about when a write completes.  With IDE
    write caching turned on, IDE hard drives will not only write data to disk
    out of order, they will sometimes delay some of the blocks indefinitely
    under heavy disk load.  A crash or power failure can result in serious
    file system corruption.  So our default was changed to be safe.  Unfortu-
    nately, the result was such a huge loss in performance that we caved in
    and changed the default back to on after the release.  You should check
    the default on your system by observing the hw.ata.wc sysctl variable.
    If IDE write caching is turned off, you can turn it back on by setting
    the hw.ata.wc loader tunable to 1.  More information on tuning the ATA
    driver system may be found in the ata(4) man page.



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