From mboxrd@z Thu Jan 1 00:00:00 1970 From: David Brown Subject: Re: Question regarding --backup-file Date: Mon, 02 May 2011 21:05:28 +0200 Message-ID: References: <000301cc08db$89293160$9b7b9420$@priv.hu> <4DBECEFA.1050203@fnarfbargle.com> <001501cc08ef$f60cc620$e2265260$@priv.hu> Mime-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-2; format=flowed Content-Transfer-Encoding: 7bit Return-path: In-Reply-To: <001501cc08ef$f60cc620$e2265260$@priv.hu> Sender: linux-raid-owner@vger.kernel.org To: linux-raid@vger.kernel.org List-Id: linux-raid.ids On 02/05/11 19:39, Peter Kovari wrote: >>> Hi all, >>> >>> I understand, that a change from RAID5 to RAID6 by adding a single disk > - >>> eg. keeping the number of data disks - requires a backup file throughout > the >>> whole reshape process. For a larger, multi-TB array this means millions > of >>> writes to the backup file, which - if i'm correct - means means millions > of >>> writes to the same physical sectors of the disk that holds the backup > file. >>> Is this not problematic? How many write operations can a typical drive >>> tolerate nowadays? (on the same sectors) > >> Lots, where Lots>= 1 and Lots< infinity. > >> I've never seen rotating media specify any form of limitation to writes. >> Have you? > > No, that's why i'm asking. > > Imho, in typical usage, write cycle counts on a certain sector may not be > that high, even on a database server. I doubt it ever goes over a few > hundred thousands during the life cycle of the hard disk. On the other hand, > a single reshape on a larger array can trigger tens of millions of write > cycles on certain sectors. Sectors do fail eventually, so I'm wondering if > the "no limit" is truly a no limit, or manufacturers just won't state this > info because in "normal" usage, customers will never reach that limit. > > Btw, i'm sure SSD's are not meant to take such a pressure. > Good large SSD's can be written to continuously for /years/ before they wear out. It can be a different matter for smaller and cheapo drives, but it's not an issue for good disks now. Suppose you have a 128 GB disk with SLC flash. Each sector is good for roughly 100,000 erase/re-write cycles (or more, if you are kind to the disk and keep it cool). Since wear-leveling spreads the writes around the disk, you can write 100,000 x 128 GB of data - at 200 MB/s continuously, that would take 2 years without a pause for breath. Even if the wear-leveling isn't perfect, and even if you substitute a cheaper MLC SSD (with 10,000 cycles), the effort of being the backup file for a raid reshape is not going to be a challenge. Also, some SSD's have super-cap backed up ram caches - writes can be safely buffered before being written. If you overwrite the same sector fast enough, it will never actually be written to the flash (until the final write, of course). For hard disks, sectors do wear out, but they tolerate a lot of writes first. And the hard disk firmware will re-locate the worn out sector transparently.