On Tue, Jun 15, 2010 at 4:20 PM, Erik Trimble <erik.trim...@oracle.com>wrote:
> On 6/15/2010 6:57 AM, Arve Paalsrud wrote: > > On Tue, Jun 15, 2010 at 3:09 PM, Erik Trimble <erik.trim...@oracle.com>wrote: > > I'd go with 2 Intel X25-E 32GB models for ZIL. Mirror them - striping isn't >> really going to buy you much here (so far as I can tell). 6Gbit/s SAS is >> wasted on HDs, so don't bother paying for it if you can avoid doing so. >> Really, I'd suspect that paying for 6Gb/s SAS isn't worth it at all, as >> really only the read performance of the L2ARC SSDs might possibly exceed >> 3Gb/s SAS. >> > > What about bandwidth in this scenario? Won't the ZIL be limited to the > throughput of only one X25-E? The SATA disks operates at 3Gb/s through the > SAS expanders, so no 6Gb/s there. > > Yes - though I'm not sure how the slog devices work when there is more than > one. I *don't* think they work like the L2ARC devices, which work > round-robin. You'd have to ask. If they're doing a true stripe, then I > doubt you'll get much more performance as weird as that sounds. Also, even > with a single X25-E, you can service a huge number of IOPS - likely more > small IOPS than can be pushed over even an Infiniband interface. The place > that the Infiniband would certainly outpace the X25-E's capacity is for > large writes, where a single 100MB write would suck up all the X25-E's > throughput capability. > But the Intel X25-E are limited to about 200 MB/s write, regardless of IOPS. So when throwing a lot of 16k IOPS (about 13 000) at it, it will still be limited to 200 MB/s - or about 6-7% of the throughput of an QDR InfiniBand links capacity. So I hereby officially ask: Can I have multiple slogs striped to handle higher bandwidth than a single device can - is that supported in ZFS? > -- > Erik Trimble > Java System Support > Mailstop: usca22-123 > Phone: x17195 > Santa Clara, CA > > - Arve
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