Re:I Want a New Data Store (for Craigslist)
by ck2 on Friday April 30, 2010 @01:30AM
(#1307318)
I just had another thought in case you check back on this thread someday.
Do you know if FusionIO drives internally run as Raid5? Not your external Raid0, but since flash chips are so relatively small in capacity right now, they must using an array internally, maybe Raid5?
If so, Raid5 has a severe performance penalty when very small amount of data are being written (smaller than a single stripe) because it has to then go update the parity across the volume on every write and that defeats any caching.
We already know ALTER itself is slow but I wonder if your storage is working against you in this case because ALTER does a single write for every row in the column when rebuilding the table...?
Do you know if FusionIO drives internally run as Raid5? Not your external Raid0, but since flash chips are so relatively small in capacity right now, they must using an array internally, maybe Raid5?
If so, Raid5 has a severe performance penalty when very small amount of data are being written (smaller than a single stripe) because it has to then go update the parity across the volume on every write and that defeats any caching.
We already know ALTER itself is slow but I wonder if your storage is working against you in this case because ALTER does a single write for every row in the column when rebuilding the table...?