Jan Engelhardt wrote:
On Mar 11 2007 22:45, Ric Wheeler wrote:
Jan Engelhardt wrote:
On Mar 11 2007 18:51, Ric Wheeler wrote:
During the recent IO/FS workshop, we spoke briefly about the
coming change to a 4k sector size for disks on linux. If I
recall correctly, the general feeling was that the impact was
not significant since we already do most file system IO in 4k
page sizes and should be fine as long as we partition drives
correctly and avoid non-4k aligned partitions.
Sorry about jumping right in, but what about an 'old-style'
partition table that relies on 512 as a unit?
I think that the normal case would involve new drives which
would need to be partitioned in 4k aligned partitions.
Shouldn't that work regardless of the unit used in the
partition table?
Assume this partition table on my current HD:
Disk /dev/hdc: 251.0 GB, 251000193024 bytes
255 heads, 63 sectors/track, 30515 cylinders
Units = cylinders of 16065 * 512 = 8225280 bytes
Device Start End Blocks Id System
/dev/hdc1 1 33 265041 82 Linux swap / Solaris
/dev/hdc2 34 30515 244846665 5 Extended
That is, 255 * 63 * 30515 * 512 == roughly 251 GB.
Now, if this disk was copied byte per byte (/bin/dd) to a
4096-based disk, and Linux would start using a sector size of
4096, then I would suddenly have
255 * 63 * 30515 * 4096 == 2 TB
Although I would not mind the 2 TB, the partition table would
read quite differently (note the Blocks column which is
multiplied by 4 (512x4=4096))
At this level, for RMW drives, nothing changes. The partition software,
ATA driver, and all other bits continue to think that sector size == 512
bytes.
The partition software /hopefully/ becomes smart enough to understand
the alignment necessary, but that is not a requirement.
This is the key to understanding the difference between a physical
(==platters) sector size change without a logical (==ATA interface)
sector size change.
Device Start End Blocks Id System
/dev/hdc1 1 33 1060164 82 Linux swap / Solaris
/dev/hdc2 34 30515 979386660 5 Extended
Which would mean that the swap partition reaches into the real
data partition and would corrupt it.
For RMW drives, RMW cycles would occur but not corruption.
For non-RMW drives, this just wouldn't occur.
Jeff
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