>> >> Since this is a SSD you're talking about, unless you have enabled >> nonvolatile write cache on that disk (which you should never do), and the >> disk incorrectly handles cache flush commands (which it should never do), >> then the supercap is irrelevant. All ZIL writes are to be done >> synchronously. >> > This SSD doesn't use nonvolatile write cache (at least i don't think it > does, it's a SF-1500 based ssd) > I might be wrong about this, but i thought one of the biggest things about > the sandforce was that it doesn't use DRAM >
I am not an expert but reading through lots of materials in the subject searching for some affordable ZIL solution I come to this: The sandforce controllers use ram for caching, but they are using limited cache on-die and does not require _external_ (to the controller) DRAM on board. Read this article about SandForce controllers (the info comes actually from SandForce as it is stated in the first paragraph): http://www.anandtech.com/show/3661/understanding-sandforces-sf1200-sf1500-not-all-drives-are-equal Especially this part: "As an enterprise class solution, the SF-1500 is designed to complete any writes in progress in the event of sudden power loss. The SF-1500 firmware is configured expecting the presence of the super cap we saw in the Vertex 2 Pro. As such it expects that it can write at full speed without any worries about power loss. In other words, it assumes you have power failure protection at the drive level." Also if you read it through you will see that different OEMs use the controllers differently, not always conforming to the guidelines provided by sandforce. So having an SF controller on board does not guarantee anything (like having a non-production ready firmware). >> >> If you have a power loss, you don't lose your pool, and you also don't >> lose >> any writes in the ZIL. You do, however, lose any async writes that were >> not >> yet flushed to disk. There is no way to prevent that, regardless of ZIL >> configuration. > > Yes, I know that i lose async writes....i just wasn't sure if that resulted > in an issue...I might be somewhat confused to how the ZIL works but i > thought the point of the ZIL was to "pretend" a write actually happened when > it may not have actually been flushed to disk yet...in this case, a write to > the zil might not make it to disk....i just didn't know if this could result > in a loss of a pool due to some sort of corruption of the uberblock or > something.....I'm not entirely up to speed on the voodoo that is ZFS. > > If your ZIL does use nonvolatile cache and does not honor flush requests then a powerloss is the same as loosing the ZIL altogether since it will not have the data saved for a playback. _______________________________________________ zfs-discuss mailing list zfs-discuss@opensolaris.org http://mail.opensolaris.org/mailman/listinfo/zfs-discuss
