From mboxrd@z Thu Jan 1 00:00:00 1970 From: Justin Piszcz Subject: Re: Basic RAID5/6 reshape question Date: Fri, 12 Dec 2008 15:27:23 -0500 (EST) Message-ID: References: <4940E377.7070907@wasp.net.au> <7d86ddb90812111706t14e7fbdar6bf31a22d510a6b6@mail.gmail.com> <4942B8DD.8090809@wasp.net.au> Mime-Version: 1.0 Content-Type: TEXT/PLAIN; charset=US-ASCII; format=flowed Return-path: In-Reply-To: <4942B8DD.8090809@wasp.net.au> Sender: linux-raid-owner@vger.kernel.org To: Brad Campbell Cc: Ryan Wagoner , RAID Linux List-Id: linux-raid.ids On Fri, 12 Dec 2008, Brad Campbell wrote: > Ryan Wagoner wrote: >> It takes 3 hours to resync my 3 x 1 TB drive RAID 5 array at a rate of >> 85MB/s. It shouldn't take 20 hours to write 3 x 1 TB drives. How fast >> is it progressing? You might consider a PCI-X or PCI Express SATA or >> SAS controller card. I have my drives connected to an Supermicro LSI >> 1068 PCI Express SAS card, which performs great. >> > > I'm using 2 x Marvell 7042 based PCI-E x4 controllers and a SIL 3xxx (I don't > recall but its PCI-E x1 and 2 ports). Bandwidth is not an issue. > > A resync appears to be a completely different kettle of fish to a reshape. > With re-shape it appears to write status data to the raid superblocks > periodically. This appears to thrash the heads from the current position to > the end of the disk and back. At the start of the reshape I was running about > 15-20MB/s to each drive. As the reshape progressed to the end of the disks I > was looking at closer to 40MB/s. So obviously the final figure was less than > the initial 20 hours, but a hell of a lot slower than a straight resync. > > This blind stab at reshape behaviour is absolutely pure conjecture based on a > gross guess at what was happening, however it does seem to bear out scrutiny. > >> The other option would be to create 2 RAID 5 arrays of 5 drives each. >> Create the first array and use LVM with the filesystem of your choice. >> Add the second array later and expand the LVM volume. > > Mmm thanks, but no.. to use a car analogy, "I'd like something I can fix with > a screwdriver and a hammer". LVM is overcomplicating things way past what I > require. In addition, I really like RAID-6. Over the last 4 years with my 15 > drive RAID-6 I have had two double drive failures. Both times one drive died > in a not so nice fashion, and one developed a grown defect during the > rebuild. Raid-5 just does not cut the mustard with me anymore. ^^^^^^^^^^^^^ Agree with you there, even when you have the data backed up losing and having to rebuild everything is such a pain-- RAID-6 has served me quite well and I have only started using it recently, it did save me from one double-drive failure so far, I have not looked back since. Justin.