Date: Wed, 16 Feb 2005 13:12:08 +0000 From: David Rhodus <sdrhodus@gmail.com> To: delphij@delphij.net Cc: Scott Long <scottl@freebsd.org> Subject: Re: Journalling FS and Soft Updates comparision Message-ID: <fe77c96b05021605124959070b@mail.gmail.com> In-Reply-To: <1108525810.676.21.camel@spirit> References: <20050210030119.GD29396@alzatex.com> <fe77c96b0502151935306e9aa1@mail.gmail.com> <1108525810.676.21.camel@spirit>
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On Wed, 16 Feb 2005 11:50:10 +0800, Xin LI <delphij@frontfree.net> wrote: > Hi, David, >=20 > =E5=9C=A8 2005-02-15=E4=BA=8C=E7=9A=84 22:35 -0500=EF=BC=8CDavid Rhodus= =E5=86=99=E9=81=93=EF=BC=9A > > > Softupdates really aren't a whole lot different from journalling. Bo= th > > > > No, journalling and soft updates are orthogonal technologies; they > > do not solve the same problem space, although there is some minor > > overlap. Soft updates is not able to solve all the problems which > > journaling can. >=20 > Would you please give some examples? I think a file system journalling > is different from what we have seen in applications, e.g. databases, > since the primary goal of having a journal is to make the file system > recoverable to last checkpoint, is that correct, or maybe I have missed > something? >=20 > Thanks! >=20 > Cheers, > -- > Xin LI <delphij delphij net> http://www.delphij.net/ Don't try and think of a filesystem as a database, in practice a filesystem is not a database. If you consider all filesystems in the context of a graph, then what you end up with is: 1.) a graph 2.) a list of node relationships 3.) a finite state automation model for node conflict resolution Though here is a example of why a journalled filesystem performs betters. With todays disk drives a DC power failure can result in an entire track getting trashed. Soft updates optimizes for sector writing not track aligned writing. Journalling can journal on the basis of track sized extents. This is one case were a journalled filesystem can recover whereas soft updates could not, from a power failure, if resolution is needed. If this were to happen on a disk protected by soft updates, damage may have accrued. --=20 -David Steven David Rhodus <drhodus@machdep.com>
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