Date: Tue, 3 Mar 1998 19:19:06 +1030 From: Greg Lehey <grog@lemis.com> To: shimon@simon-shapiro.org Cc: hackers@FreeBSD.ORG, blkirk@float.eli.net, jdn@acp.qiv.com, wilko@yedi.iaf.nl, tlambert@primenet.com, sbabkin@dcn.att.com Subject: Re: SCSI Bus redundancy... Message-ID: <19980303191906.19116@freebie.lemis.com> In-Reply-To: <XFMail.980302174819.shimon@simon-shapiro.org>; from Simon Shapiro on Mon, Mar 02, 1998 at 05:48:19PM -0800 References: <19980303084608.56831@freebie.lemis.com> <XFMail.980302174819.shimon@simon-shapiro.org>
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On Mon, 2 March 1998 at 17:48:19 -0800, Simon Shapiro wrote:
>
> On 02-Mar-98 Greg Lehey wrote:
>
> ...
>
>> That's not the point. OK, we were talking about RAID 5 here, which
>> also has parity blocks, but the point is that if you add another disk,
>> you're effectively adding another block every n blocks in the file
>> system address space. It requires some non-trivial data movement to
>> rearrange all the data (more specifically, except for the first n (n =
>> old number of drives) blocks, you must move *everything*, and you must
>> recalculate parity for every stripe.
>
> Not quite. The [parity is not in the filesystem. It is in the ``device''.
> The filesystem sees a plain, old LBA addressable ``disk''. If a RAID-5
> array grows, the ``disk'' will grow by having its last block address be
> (old_size - 1) + increment.
Yes, of course. Sorry for my sloppy terminology.
>> My question ("How can that work?") was based on the misassumption that
>> this would be too much work to be justifiable.
>
> ``Justifiable'' is a relative term. If the cost is 30% reduction in
> perfromance vs. shutdown of service for 2 hours, that may be real cheap.
> Some of the systems we work on measure downtime in minutes/year, and number
> of shutdowns in once/several_years. In that scenario, a customer may find
> this ability, as complex as it may be, quite attractive.
Agreed.
Greg
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