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}...............:
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This archive was generated by hypermail 2b29 : Wed Aug 07 2002 - 22:00:28 EST