Date: Tue, 18 May 2004 10:40:20 +0800 From: Xin LI <delphij@frontfree.net> To: Gary Corcoran <garycor@comcast.net> Cc: Julian Elischer <julian@elischer.org> Subject: Re: QMail and SoftUpdates Message-ID: <20040518024020.GA631@frontfree.net> In-Reply-To: <40A974DA.30704@comcast.net> References: <20040517174836.GA983@frontfree.net> <Pine.BSF.4.21.0405171323010.27448-100000@InterJet.elischer.org> <20040518021433.GA969@frontfree.net> <40A974DA.30704@comcast.net>
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--VS++wcV0S1rZb1Fb Content-Type: text/plain; charset=us-ascii Content-Disposition: inline Content-Transfer-Encoding: quoted-printable On Mon, May 17, 2004 at 10:28:42PM -0400, Gary Corcoran wrote: > Xin LI wrote: [snip] > >doing fsync(), because with ATA hardware writing cache, the disk will > >"cheat" the opearting system - Before the data actually goes to disk, > >it tells that it is, and data is lost if crash occours here. > ^^^^^ > Umm - shouldn't it be only if *power loss* occurs here? > Even if the OS crashes, as long as power is supplied to the drive, > its firmware should finish writing the data from its cache to the > disk media, no? And therefore, as long as one has a stable power > source, e.g. running off a UPS, there really isn't any great risk > from on-drive write caches, is there? I don't think so. Personally I think it is still danger if a crash occours. Soft Updates rolls back buffers when necessary to guarantee the on-disk state is in a "recoverable consistency". With a drive cheating the operating system and reorder operating system write operations, there is a potential risk that, when operating system "thinks" that "this order is good and won't cause any problem", the disk "thinks" that "that order is better and I write in that order", and an "unexpected SoftUpdates inconsistency" occours. This is very likely to cause data loss, or even more worse problem because there might be wild pointers in i-nodes, or used blocks that are not marked "used" in the cylgroup bitmap. Cheers, --=20 Xin LI <delphij frontfree net> http://www.delphij.net/ See complete headers for GPG key and other information. --VS++wcV0S1rZb1Fb Content-Type: application/pgp-signature Content-Disposition: inline -----BEGIN PGP SIGNATURE----- Version: GnuPG v1.2.4 (FreeBSD) iD8DBQFAqXeUOfuToMruuMARAjgqAJ90DK7u2CEeRi8ji3hGwWiOGR1XHQCfbaXk iLWHLLhAuaz4Rn5ja0IByYA= =o3IG -----END PGP SIGNATURE----- --VS++wcV0S1rZb1Fb--
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