[CentOS] raid resync speed? - laptop drive-

[Les Mikesell]

Robert Nichols wrote:
> On 05/22/2010 07:39 PM, Robert Nichols wrote:
>> On 05/22/2010 05:46 PM, Les Mikesell wrote:
>>> Does the 4K sector size mean that the drive is going to read the 4K chunk then
>>> merge in the 512 bytes you wrote, the wait for the sector come around again to
>>> write it back?  I guess that could explain the 10x write speed difference
>>> regardless of cylinder alignment.   Read speed doesn't seem that much different.
>> Yes, that's exactly what it means.  Every unaligned write or write that is
>> not a multiple of the 4KB sector size becomes a read-modify-write within the
>> drive, and a 10X reduction in write throughput is typical.
> 
> I should add that the kernel normally will do I/O in multiples of its 4KB
> (typical) page size where possible, but I have no idea whether any effort
> is made to align those writes if the drive does not report a 4KB physical
> sector size, or whether it even makes sense to try beyond what the
> elevator algorithm does for coalescing sequential writes.
> 
> I don't currently have any of these "enhanced format" drives, nor am I
> using RAID, so all I can report is the collected experience of others.

Well, the form factor is certainly nice.  I got a hot-swap carrier with 2 slots 
that fits in a floppy bay and the drives themselves are tiny so it seemed ideal 
for copies of data to go offsite.  I just wish it would work...  Even a dd at 
the disk level seems slow so I'm not sure the writes are being aggregated even 
if you ignore partitioning and offsets.

-- 
   Les Mikesell
    lesmikesell at gmail.com