Date: Mon, 20 Feb 2012 15:20:54 -0700 From: Scott Long <scottl@samsco.org> To: Alex Samorukov <ml@os2.kiev.ua> Cc: freebsd-stable@freebsd.org, Peter Jeremy <peterjeremy@acm.org> Subject: Re: disk devices speed is ugly Message-ID: <DF15DE9F-F145-4BAD-A8C0-B74033739207@samsco.org> In-Reply-To: <4F429DFB.8050003@os2.kiev.ua> References: <4F215A99.8020003@os2.kiev.ua> <4F27C04F.7020400@omnilan.de> <4F27C7C7.3060807@os2.kiev.ua> <CAJ-VmomezUWrEgxxmUEOhWnmLDohMAWRpSXmTR=n2y_LuizKJg@mail.gmail.com> <4F37F81E.7070100@os2.kiev.ua> <CAJ-Vmok9Ph1sgFCy6kNT4XR14grTLvG9M3JvT9eVBRjgqD%2BY9g@mail.gmail.com> <4F38AF69.6010506@os2.kiev.ua> <20120213132821.GA78733@in-addr.com> <20120214200258.GA29641@server.vk2pj.dyndns.org> <CA28336C-8462-4358-9E68-B01EEB4237CE@samsco.org> <4F429DFB.8050003@os2.kiev.ua>
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On Feb 20, 2012, at 12:24 PM, Alex Samorukov wrote: > On 02/15/2012 05:50 AM, Scott Long wrote: >>=20 >> What would be nice is a generic caching subsystem that any FS can use >> - similar to the old block devices but with hooks to allow the FS to >> request read-ahead, advise of unwanted blocks and ability to flush >> dirty blocks in a requested order with the equivalent of barriers >> (request Y will not occur until preceeding request X has been >> committed to stable media). This would allow filesystems to regain >> the benefits of block devices with minimal effort and then improve >> performance& cache efficiency with additional work. >>=20 >> Any filesystem that uses bread/bwrite/cluster_read are already using = the "generic caching subsystem" that you propose. This includes UDF, = CD9660, MSDOS, NTFS, XFS, ReiserFS, EXT2FS, and HPFS, i.e. every local = storage filesystem in the tree except for ZFS. Not all of them = implement VOP_GETPAGES/VOP_PUTPAGES, but those are just optimizations = for the vnode pager, not requirements for using buffer-cache services on = block devices. As Kostik pointed out in a parallel email, the only = thing that was removed from FreeBSD was the userland interface to cached = devices via /dev nodes. This has nothing to do with filesystems, though = I suppose that could maybe sorta kinda be an issue for FUSE?. > May be its possible to provide some generic interface for fuse based = filesystems to use this generic cache? I can test it and report = performance. >=20 What you're asking for is to bring back the cached raw devices. I don't = have a strong opinion on this one way or another, except that it's a = pretty specific use case. Does the inherent performance gap with user = land filesystems warrant this? Maybe a simple cache layer can be put = into FUSE that would allow client filesystems the same control over = block caching and clustering that is afforded in the kernel? Scott
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