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    SubjectRe: [PATCH] 2.4.10 improved reiserfs a lot, but could still be better
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    On Monday 24 September 2001 05:11 pm, Matthias Andree wrote:
    > On Mon, 24 Sep 2001, Nicholas Knight wrote:
    > > Would you like to read the rest of my message please? Cheap UPS's
    > > can provide protection against power failures. If your data is that
    > > valuble, you can afford a cheap UPS to give you 5 minutes to shut
    > > down.
    >
    > No UPS can protect you from system crashes. The problem is, with the
    > drive cache on, the drive will acknowledge having written the data
    > early and reorder its writes, but who makes guarantees it can write
    > its whole 2 MB to disk should the power fail? No-one. ATA6 drafts
    > have a NOTE that says, the FLUSH CACHE command may take longer than
    > 30 s to complete.
    >
    > Journalling File systems don't get you anywhere if the drive reorders
    > its blocks before the write (I presume, most will do), they may
    > instead turn the whole partition to junk without notice, because any
    > assumptions as to the on-disk structure don't hold.
    >
    > > > Linear writing as dd mostly does is BTW something which should
    > > > never be affected by write caches.
    > >
    > > Explain the numbers then.
    >
    > I can't, any explanation right now would be conjecture. I can
    > reproduce the numbers on my IBM DTLA-307045 (Promise) and on my
    > Western Digital CAC420400D (VIA KT133, the disk looks like an IBM
    > DJNA-352030 OEM, though).
    >
    > However, would you care to elaborate how switching OFF the cache
    > should harm data, provided you don't need to cater for power outages
    > (UPS attached, e. g.)?

    It's a very remote possability of failure, like most instances where
    write-cache would cause problems. Catastrophic failure of the IDE cable
    in mid-write will cause problems. If write cache is enabled, the write
    stands a higher chance of having made it to the drive before the cable
    died, with it off, it stands a higher chance of NOT having made it
    entirely to the drive.
    For most drives, I don't know for sure if they'd finish the write
    that's now sitting in their cache, but I expect higher quality drives
    (such as our IBM drives) definitely would. Infact I may even be willing
    to test this later (my swap partition looks like it wants to help :)

    >
    > hdparm:
    >
    > " -W Disable/enable the IDE drive's write-caching fea­
    > ture (usually OFF by default)."
    >
    > > I followed *YOUR* instructions for disabling write caching.
    >
    > No-one doubts you did. I said it's weird that the drive write cache
    > has an impact on dd figures. It may be worthwhile to investigate
    > this, but again, any try to explain this would be a guess.
    >
    > It may be an implementation problem in our IBM drives which ship with
    > their write caches enabled, someone please do this test on current
    > Fujitsu, Maxtor or Seagate IDE drives or with different controllers.

    Either Maxtor or Western Digital share very close designs to IBM
    drives, I belive they had some sort of development partnership. I'm not
    sure if it was Maxtor or WD.

    >
    > It would suffice if the kernel could flush the drive's buffers on
    > fsync() and other synchronous operations, but a flush command has
    > only recently appeared in the ATA standards, as it seems. I only have
    > drafts here, ATA 3 draft rev. 6 did not offer any command to flush
    > the cache, ATA 6 draft makes it mandatory for all devices that do
    > offer a PACKET interface. Not sure about the actual ATA 3, 4, or 5
    > standards.
    >
    > Why are disk drives slower with their caches disabled on LINEAR
    > writes?

    Maybe the cache isn't doing what we think it is?
    You're right, now that I'm thinking about it, it doesn't make a whole
    lot of sense. The cache on our IBM's is just 2MB.
    Does anyone have contacts at IBM and/or Western Digital? Something's
    up... The 256MB write with write-cache off was going at 5.8MB/sec, and
    with it on it was going at 14.22MB/sec (averages). One interesting
    thing, the timings are showing a pretty consistant but tiny increase in
    sys time with write caching on.
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