[CentOS] raid resync speed? - laptop drive-

[John R Pierce]

Les Mikesell wrote:
> On 5/26/2010 9:52 AM, John R Pierce wrote:
>   
>>>> Is that one of those WD drives that falsely
>>>> reports its physical sector size as 512 bytes?
>>>>
>>>>         
>>>  From the Scorpio blue specs, if I divide the capacity by the number of sectors, I get 512...
>>>
>>>       
>>
>> all these new 'advanced' drives look to the host like they have 512 byte
>> sectors.... its just that they pack 8 of them internally into a single
>> 4K sector.    if you don't ensure that your partitions start on a 4K (8
>> sector) boundary, then committed random writes are very slow as your
>> logical file system blocks will span multiple physical sectors
>>     
>
> How can they ever be fast if the OS is writing 512 byte sectors?  The 
> drive is going to have to read the 4k sector, merge the update, wait for 
> the disk to spin around and write it back.  The read speed seems a match 
> for a desktop Seagate with the same capacity, but writes are about 10x 
> slower, even if I dd to the raw disk which should bypass any partition 
> alignment issues.  And unfortunately since I want to store backups on 
> it, the write speed is what matters.
>
>   


well, as long as the writes aren't being 'committed' on every sector, 
they should be cached long enough for the full 4K block to be filed 
prior to actually writing to disk.


your `dd` command, what did you specify for the block size?   try 
something realistic like 32768