Date: Mon, 02 Mar 1998 17:48:19 -0800 (PST) From: Simon Shapiro <shimon@simon-shapiro.org> To: Greg Lehey <grog@lemis.com> 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: <XFMail.980302174819.shimon@simon-shapiro.org> In-Reply-To: <19980303084608.56831@freebie.lemis.com>
next in thread | previous in thread | raw e-mail | index | archive | help
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.
> 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.
----------
Sincerely Yours,
Simon Shapiro
Shimon@Simon-Shapiro.ORG Voice: 503.799.2313
To Unsubscribe: send mail to majordomo@FreeBSD.org
with "unsubscribe freebsd-hackers" in the body of the message
Want to link to this message? Use this URL: <https://mail-archive.FreeBSD.org/cgi/mid.cgi?XFMail.980302174819.shimon>