Date: Fri, 27 Mar 2015 22:57:05 +0200 From: Alexander Motin <mav@FreeBSD.org> To: freebsd-fs@FreeBSD.org, "freebsd-hackers@freebsd.org" <freebsd-hackers@freebsd.org> Subject: MAXBSIZE increase Message-ID: <5515C421.4040703@FreeBSD.org>
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Hi. Experimenting with NFS and ZFS I found an inter-operation issue: ZFS by default uses block of 128KB, while FreeBSD NFS (both client and server) is limited to 64KB requests by the value of MAXBSIZE. On file rewrite that limitation makes ZFS to do slow read-modify-write cycles for every write operation, instead of just writing the new data. Trivial iozone test show major difference between initial write and rewrite speeds because of this issue. Looking through the sources I've found and in r280347 fixed number of improper MAXBSIZE use cases in device drivers. After that I see no any reason why MAXBSIZE can not be increased to at least 128KB to match ZFS default (ZFS now supports block up to 1MB, but that is not default and so far rare). I've made a test build and also successfully created UFS file system with 128KB block -- not sure it is needed, but seems it survives this change well too. Is there anything I am missing, or it is safe to rise this limit now? -- Alexander Motin
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