From mboxrd@z Thu Jan 1 00:00:00 1970 From: Ryusuke Konishi Subject: Re: Split some metadata onto separate device? Date: Wed, 08 Jun 2011 22:49:06 +0900 (JST) Message-ID: <20110608.224906.85409768.ryusuke@osrg.net> References: <20110608.214852.237935973.ryusuke@osrg.net> <20110608.215803.224753719.ryusuke@osrg.net> <201106081508.39345.dexen.devries@gmail.com> Mime-Version: 1.0 Content-Transfer-Encoding: 7bit Return-path: In-Reply-To: <201106081508.39345.dexen.devries-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org> Sender: linux-nilfs-owner-u79uwXL29TY76Z2rM5mHXA@public.gmane.org List-ID: Content-Type: Text/Plain; charset="iso-2022-jp" To: dexen.devries-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org Cc: linux-nilfs-u79uwXL29TY76Z2rM5mHXA@public.gmane.org On Wed, 8 Jun 2011 15:08:37 +0200, dexen deVries wrote: > Hi again, > > > On Wednesday 08 of June 2011 14:58:03 you wrote: > > On Wed, 08 Jun 2011 21:48:52 +0900 (JST), Ryusuke Konishi wrote: > > > On Fri, 3 Jun 2011 19:42:28 +0200, dexen deVries wrote: > > > > By the way, it seems to me that the in-kernel GC does not attempt to > > > > de- fragment files. Is that the case? If so, would probably mean > > > > extents would not help very much for fragmented files. Also, on > > > > magnetic media, a lot of seeking happens in case of fragmented > > > > files. > > > > > > Yes, but extents will decrease the amount of metadata, and this has > > > potential for speed up. > > > > > > NILFS2 uses 32 bytes metadata per disk block at present. I guess you > > > know that the number of DAT blocks are actually indispensable through > > > analysis using the dumpseg tool. > > > > Oh, looks confusing. I meant the amount of DAT blocks is not > > negligible. > > Understood ;-) > > Anyway, isn't some fragmentation avoidance necessary to profit from extents > (that we hope we'll use at some point)? I guess it doesn't become a big issue unless we do many small random writes like database. > AFAIK, an extent is only good to describe file blocks if they are laid out > continuous on the block device. Yep. And, we know files are enough continuous in most cases. We use at least 16 bytes per 4 kiro-bytes disk block to point the block (16KB for 8MB segment); btrees of nilfs2 use an 8 bytes key and an 8 bytes pointer for a disk block. In typical case, this can be reduced to 16 bytes per file (16 bytes for 8MB segment) with the extents. IMO, 64-bit filesystems have a reasonable reason to adopt extents though nilfs2 did not apply it. I guess applying extents to the DAT file is especially effective. Regards, Ryusuke Konishi > Cheers, > -- > dexen deVries > > [[[↓][→]]] > > For example, if the first thing in the file is: > > an XML parser will recognize that the document is stored in the traditional > ROT13 encoding. > > (( Joe English, http://www.flightlab.com/~joe/sgml/faq-not.txt )) > -- > To unsubscribe from this list: send the line "unsubscribe linux-nilfs" in > the body of a message to majordomo-u79uwXL29TY76Z2rM5mHXA@public.gmane.org > More majordomo info at http://vger.kernel.org/majordomo-info.html -- To unsubscribe from this list: send the line "unsubscribe linux-nilfs" in the body of a message to majordomo-u79uwXL29TY76Z2rM5mHXA@public.gmane.org More majordomo info at http://vger.kernel.org/majordomo-info.html