Re: [OT] Hard Drive Energy Not Worth Conserving drives?



Stefan Monnier put forth on 1/11/2011 10:28 PM:

Isn't it rather than the kernel chooses to only use the logical
sector size? Where/when does the drive report 512B physical
sector sizes?

Read the ATA and SCSI specifications. Or ask on either mailing list. In short,
the drive presents its LBA addressing based on 512B sectors. The kernel can't
choose to ignore that--it's stuck with it. Since the drive is presenting LBA
based on 512B sectors, there is no way the kernel can address LBA based on 4K
sectors.

In any case, the issue is probably not really in the kernel but in the
filesystems and partitioning tools: all that's really needed to use the

The problem is none of those things. The problem is hybrid drives. IIRC, the
kernel, libata, libscsi, etc, are all ready for _native_ 4K sector drives. The
current "problem" with the hybrid drives is that the partitioning utilities
don't automatically align partitions on the underlying 4k sector boundaries.
So, what happens is, in essence, when a filesystem is laid down that uses 4K
blocks, it will lay across 4 _translated_ 512B sectors. Thus, you end up with a
4K filesystem block that lays across two physical 4K disk sectors. Thus, each
time that FS block is written, 8*512B sectors must be read and re-written.
Thus, _TWO_ physical 4K disk sectors must be read and re-written instead of just
one on an aligned partition, akin to the the RAID5/6 read/modify/write penalty.
This misalignment cuts performance by half or more because every FS block write
requires to reads and two writes instead of just one.

drive efficiently is for fdisk/parted and for mkfs to be told (and make
use of) the physical block size. Of course, maybe a good way to provide
this info is to teach the kernel about it so those tools don't need to
use side-band info via hdparm.

Maybe you should hit the LKML, GNU fdisk, and GNU parted list archives before
continuing this thread.

Indeed, and for that reason 4KB physical blocks wouldn't cause
additional disk space usage.

The space savings with 4KB sectors has nothing to do with file systems or user
data. It has to do with the per sector ECC information that all drives
calculate and store in between sectors to safeguard the data in the sector. The
number of bits of ECC required per 4KB sector is significantly less than that
occupied by the 4 ECC segments of four 512 byte sectors. This is the ONLY
reason these 4KB sector drives were developed: more actual end user space on
the drive.

--
Stan



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