From mboxrd@z Thu Jan 1 00:00:00 1970 From: Neil Brown Subject: Re: recommendations for stripe/chunk size Date: Thu, 7 Feb 2008 16:31:11 +1100 Message-ID: <18346.38815.503023.991827@notabene.brown> References: <20080205182421.GA32250@rap.rap.dk> <47AA08E7.5000801@tmr.com> Mime-Version: 1.0 Content-Type: text/plain; charset=iso-8859-1 Content-Transfer-Encoding: QUOTED-PRINTABLE Return-path: In-Reply-To: message from Bill Davidsen on Wednesday February 6 Sender: linux-raid-owner@vger.kernel.org To: Bill Davidsen Cc: Keld Jorn Simonsen , linux-raid@vger.kernel.org List-Id: linux-raid.ids On Wednesday February 6, davidsen@tmr.com wrote: > Keld J=F8rn Simonsen wrote: > > Hi > > > > I am looking at revising our howto. I see a number of places where = a > > chunk size of 32 kiB is recommended, and even recommendations on > > maybe using sizes of 4 kiB.=20 > > > > =20 > Depending on the raid level, a write smaller than the chunk size caus= es=20 > the chunk to be read, altered, and rewritten, vs. just written if the= =20 > write is a multiple of chunk size. Many filesystems by default use a = 4k=20 > page size and writes. I believe this is the reasoning behind the=20 > suggestion of small chunk sizes. Sequential vs. random and raid level= =20 > are important here, there's no one size to work best in all cases. Not in md/raid. RAID4/5/6 will do a read-modify-write if you are writing less than one *page*, but then they often to read-modify-write anyway for parity updates. No level will every read a whole chunk just because it is a chunk. To answer the original question: The only way to be sure is to test your hardware with your workload with different chunk sizes. But I suspect that around 256K is good on current hardware. NeilBrown - To unsubscribe from this list: send the line "unsubscribe linux-raid" i= n the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html