Messages in this thread |  | | | From | Itai Nahshon <> | | Subject | Re: Disk (block) write strangeness | | Date | Wed, 7 Aug 2002 14:43:30 +0300 |
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On Monday 05 August 2002 21:49 pm, Jakob Oestergaard wrote: > 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!
Integers are 32 bit, so a 512 byte disk block contains 128 such integers... Indeed, All the values above are divisible by 128, so you have: 11668992/128 = 91164 blocks of "0x00000002" 231168/128 = 1806 blocks of "0x00000003" 1174528/128 = 9176 blocks of "0x00000002" 32512/128 = 254 blocks of "0x00000003" This does not prove, neither disprove anything about your main concern, that writes are non-atomic in the block level.
> > 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... >
I would not expect writes to be in order. A simple elevator algorithm could write fragments (cylinder sized?) in reverse order. On-disk write scheduling could start writing at any sector (to minimize rotational latency).
Knowing the disk geometry and parameters could help with understanding your results.
> 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,
-- Itai
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