Date: Tue, 3 Mar 1998 08:46:08 +1030 From: Greg Lehey <grog@lemis.com> To: sbabkin@dcn.att.com, tlambert@primenet.com Cc: shimon@simon-shapiro.org, wilko@yedi.iaf.nl, jdn@acp.qiv.com, blkirk@float.eli.net, hackers@FreeBSD.ORG Subject: Re: SCSI Bus redundancy... Message-ID: <19980303084608.56831@freebie.lemis.com> In-Reply-To: <C50B6FBA632FD111AF0F0000C0AD71EE4132C8@dcn71.dcn.att.com>; from sbabkin@dcn.att.com on Mon, Mar 02, 1998 at 02:23:50PM -0500 References: <C50B6FBA632FD111AF0F0000C0AD71EE4132C8@dcn71.dcn.att.com>
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On Mon, 2 March 1998 at 14:23:50 -0500, sbabkin@dcn.att.com wrote:
>> ----------
>> From: Terry Lambert[SMTP:tlambert@primenet.com]
>>
>>>>> I think Julian's SLICE code has something in that direction.
>> DPT
>>>>> supports INCREASING the size of a RAID-5 array by adding drives.
>>>>
>>>> How can that work?
>>>
>>> Something like
>>> - read N RAID blocks from K disks
>>> - compute new checksum for K+1 disks and write as less number
>>> of RAID blocks but each one of bigger size (K+1/K times)
>>> - add empty blocks at the end of RAID in the added space
>>
>> You would have to remember to grab the blocks to be relocated with
>> the same O(n) randomness as their allocation. 8-).
>>
> Huh ? Probably I've missed something about RAIDs. I've thought
> that, for example, RAID block 0 consists of blocks 0 of all
> the physical disks. And so on. And I've thought that RAID itself
> does not allocate any blocks, the upper level like filesystem or
> volume manager does it, RAID just makes chechsuming. Am I wrong again ?
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.
My question ("How can that work?") was based on the misassumption that
this would be too much work to be justifiable.
Greg
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