Re: The danger of dishonest disk drives (WAS:Re: Need to remove a ghost file, but can't because it doesn't exist)
On Wed, Nov 22, 2006 at 10:52:07AM -0500, hendrik@topoi.pooq.com wrote:
> On Tue, Nov 21, 2006 at 10:38:45AM -0500, Douglas Tutty wrote:
> >
>
> > So I use JFS for everything.
> >
> I looked a dmesg output, and in the midle it says:
>
> [ 31.547456] Probing IDE interface ide0...
> [ 31.837706] hda: WDC WD2500JB-00GVC0, ATA DISK drive
> [ 32.511260] ide0 at 0x1f0-0x1f7,0x3f6 on irq 14
> [ 32.511467] Probing IDE interface ide1...
> [ 32.800442] hdc: WDC WD2500JB-00GVC0, ATA DISK drive
> [ 33.247857] hdd: HL-DT-ST DVDRAM GSA-4167B, ATAPI CD/DVD-ROM drive
> [ 33.307431] ide1 at 0x170-0x177,0x376 on irq 15
> [ 33.319500] hda: max request size: 1024KiB
> [ 33.330796] hda: 488397168 sectors (250059 MB) w/8192KiB Cache, CHS=30401/255/63, UDMA(100)
> [ 33.332722] hda: cache flushes supported
> [ 33.332762] hda: hda1 hda2 hda3
> [ 33.351996] hdc: max request size: 1024KiB
> [ 33.368251] hdc: 488397168 sectors (250059 MB) w/8192KiB Cache, CHS=30401/255/63, UDMA(33)
> [ 33.370187] hdc: cache flushes supported
> [ 33.370204] hdc: hdc1 hdc3
> [ 33.381946] hdd: ATAPI 48X DVD-ROM DVD-R-RAM CD-R/RW drive, 2048kB Cache, UDMA(33)
>
> Do you have similar lines in your dmesg output?
>
> The key words here are "cache flushes supported", suggesting to me that
> it is indeed possible for a file system to flush the cache, creating
> safe points in the journalling. Does anyone know if my interpretation
> is correct?
>
> The remaining questin is whether these cache flushes are indeed carried
> out at the right moments.
>
> I've found the following references to suggest that some attempt is
> being made to so this, but the exact status is unclear to me:
>
> http://oss.sgi.com/projects/xfs/faq.html
> http://lwn.net/Articles/157209/
>
> The latter of the two is a commit message that commits changes to a
> file called
> blk-fixes/Documentation/block/barrier.txt
> Now if I could find the rest of that file, things might be clearer.
Hi Hendrik,
The faq link you gave is for xfs which is defferent from jfs.
I have no lines on either of my boxes mentioning cache flushes. Then
again, I found WD drives unreliable for some reason and perhaps this is
why. Now, I only use Seagate.
The best source of jfs information is from ibm. The easiest way to
locate this is with google:
jfs site:ibm.com
Although since IBM likes to move stuff around, google often can't find
things. So use the search function on IBM's website at www.ibm.com
Also, today, I'm not getting a response from ibm.com/developerworks,
which is where most of the linux jfs documentation is.
Note that IBM does say that jfs focus more on being able to restore
filesystem integrity than on data integrity. For higher integrity they
suggest the sync mount option.
What I haven't seen covered is how the fs/lvm/raid1 interact re the fs
committing the journal to disk. There's probably a 2.6 kernel internals
document that discusses this but I haven't seen it yet.
I wonder if there is a filesystem designed from the outset for data
integrity after power-failure.
Doug.
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