Messages in this thread | | | Date | Mon, 5 Aug 2002 20:49:21 +0200 | From | Jakob Oestergaard <> | Subject | Disk (block) write strangeness |
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Hello all,
While investigating how various disks handle power-loss during writes, I came across something *very* strange.
It seems that
*) Either the disk writes backwards (no I don't believe that) *) Or the kernel is writing 256 B blocks (AFAIK it can't) *) The disk has some internal magic that cause a power-loss during a full block write to leave the first half of the block intact with old data, and update the second half of a block correctly with new data. (And I don't believe that either).
The scenario is: I wrote a program that will write a 50 MB block with O_SYNC to /dev/hdc. The block is full of 32-bit integers, initialized to 0. For every full block write (the block is written with one single write() call), the integers are incremented once.
So first I have 50 MB of 0's. Then 50 MB of 1's. etc.
During this write cycle, I pull the power cable. I get the machine back online and I dump the 50 MB block.
What I found was a 50 MB block holding: 11668992 times "0x00000002" 231168 times "0x00000003" 1174528 times "0x00000002" 32512 times "0x00000003"
Please note that 32512 is *not* a multiple of 512. And please note that the 3's are written *after* the 2's, so actually there is a 512 byte block on the disk which contains 2's in the first half, and 3's in the second half!
How on earth could that happen ?
Why does the kernel not write from beginning to end ? Or why doesn't the disk ?
And does the elevator cause the writes to be shuffled around like that - I would have expected the kernel to write from beginning to end every single time...
The kernel is 2.4.18 on some i686 box The disk is a Quantum Fireball 1GB IDE (from way back then ;) The IDE chipset is an I820 Camino 2
I can submit the test program or do further tests, if anyone is interested.
Thank you,
-- ................................................................ : jakob@unthought.net : And I see the elder races, : :.........................: putrid forms of man : : Jakob Østergaard : See him rise and claim the earth, : : OZ9ABN : his downfall is at hand. : :.........................:............{Konkhra}...............: - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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