Re: Sharing disks amoung multiple software RAIDs
From: Bill Davidsen <hidden>
Date: 2008-05-02 21:17:07
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Alex Davis wrote:
quoted hunk ↗ jump to hunk
--- On Thu, 5/1/08, Justin Piszcz <jpiszcz@lucidpixels.com> wrote:quoted
From: Justin Piszcz <redacted> Subject: Re: Sharing disks amoung multiple software RAIDs To: "Alex Davis" <redacted> Cc: linux-raid@vger.kernel.org, linux-kernel@vger.kernel.org Date: Thursday, May 1, 2008, 8:50 AM On Thu, 1 May 2008, Alex Davis wrote:quoted
Is this a bad thing? I'm guessing that it is, butI want independentquoted
confirmation before I spoke to someone I knowwho's doing this.quoted
ThanksWhat is the use case, why would you want to do that? I have seen people on the list do it before, for example are you going to be utilizing both raids at the same time?Yes.quoted
If so, I would advise against it. What is the reasoning?No, I don't want to do this. I know someone who is, and I wanted to get more input before I advised them to get more disks. The RAIDs are running in degraded mode, so they'll need more disks anyway. Since they are (or hopefully soon will be) buying more disks, I'll advise them to get dedicated disks for each RAID.
Depending on the use, dedicated disk may not be better, unless the budget is large. I ran an application which had a heavily read database and a large collection of files thich were read based on offsets read from the database. I have a limited number of drives available (rackspace limit, not $). I partitioned the drives with a small partition for the heavily read database, using three copies raid1, and raid5 for the more lightly used data, across the same disks. I tried almost every layout possible with six drives, and spreading the required head motion to all drives was a big win on the heavily read database, while spreading the storage over all drives was required because of capacity. And split and shared the performance was optimized. And it did stay up with a drive fail, although "up" means "didn't lose data" rather than "usefully fast." -- Bill Davidsen [off-list ref] "Woe unto the statesman who makes war without a reason that will still be valid when the war is over..." Otto von Bismark