[Lustre-discuss] OST size limitation

[Kevin Van Maren]

On Nov 3, 2011, at 12:40 PM, Andreas Dilger wrote:
> 
> Not only is the seeking evil (talk to Kevin if you want to run 24TB OSTs on
> flash :-), but the 512-byte sector offset added by the partition table will cause all IO to be misaligned to the underlying device.


It is possible to align partition boundaries, but it is not the default.  Partitions (if used) should normally be aligned to a multiple of the RAID stripe size, although note that some RAID controllers internally compensate for the "expected" misalignment.

See http://wikis.sun.com/display/Performance/Aligning+Flash+Modules+for+Optimal+Performance


> Even with flash storage it is much better to align the IO on power-of-two
> boundaries, since the erase blocks cause extra latency if there are read-
> modify-write operations.


That also depends on the flash.  The Fusion-io products have no alignment issues.

Kevin


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