O_SYNC behavior?
End-user question:
NFS client accessing an NFS export on an XFS based filesystem via "sync" (O_SYNC) mode.
Will write confirmation be sent back from the NFS server when the data is written to the xfs journal, or not until it is actually written to disk (assuming in this case the journal is on a separate SSD device or something similar)?
Thanks,
Ray
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Quoted 7 lines · ----- "Ray Van Dolson
End-user question:
NFS client accessing an NFS export on an XFS based filesystem via "sync" (
O_SYNC) mode.Will write confirmation be sent back from the NFS server when the data is written to the xfs journal,
Data is never written to the journal.
cheers.
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Quoted 5 lines · Nathan Scott
Quoted 2 lines · ----- "Ray Van Dolson
End-user question:
Quoted 7 lines
Quoted 5 lines
NFS client accessing an NFS export on an XFS based filesystem via "sync" (
O_SYNC) mode.Will write confirmation be sent back from the NFS server when the data is written to the xfs journal,
Data is never written to the journal.
I don't think that answers the intented question.
I guess the correct answer is:
It depends on the server-side configuration.
An "async" export gets the "done"-answer immediatly.
An "sync" export (only when also mounted "sync" AFAIU) waits at least until the write command is send down the stack, maybe even until the data actually hit the platters or silicon in the SSD-case.
man 5 exports:
- snip -
sync: Reply to requests only after the changes have been committed to stable storage
- snip -
But i guess the performance will be horrible when both side are "sync".
Bis denn
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Quoted 9 lines · Matthias Schniedermeyer
I don't think that answers the intented question.
I guess the correct answer is:
It depends on the server-side configuration.
An "async" export gets the "done"-answer immediatly.
An "sync" export (only when also mounted "sync" AFAIU) waits at least until the write command is send down the stack, maybe even until the data actually hit the platters or silicon in the SSD-case.
Quoted 6 lines
man 5 exports:
- snip -
sync: Reply to requests only after the changes have been committed to stable storage
- snip -But i guess the performance will be horrible when both side are "sync".
The client side (that is mount-side) sync means implying O_SYNC for all I/O - no need for that. And sync exports are the default for every sane server.
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Quoted 15 lines · Christoph Hellwig
Quoted 14 lines · Matthias Schniedermeyer
I don't think that answers the intented question.
I guess the correct answer is:
It depends on the server-side configuration.
An "async" export gets the "done"-answer immediatly.
An "sync" export (only when also mounted "sync" AFAIU) waits at least until the write command is send down the stack, maybe even until the data actually hit the platters or silicon in the SSD-case.man 5 exports:
- snip -
sync: Reply to requests only after the changes have been committed to stable storage
- snip -
Quoted 6 lines
Quoted 1 line
But i guess the performance will be horrible when both side are "sync".
The client side (that is mount-side) sync means implying
O_SYNCfor all I/O - no need for that. And sync exports are the default for every sane server.
Thanks guys. I was asking this from the perspective of a ZFS type setup with its intent log (ZIL) on SSD's or some battery backed NVRAM.
In this setup, with sync mode NFS, acknowledgement is sent to the client as soon as the data is written to the ZIL -- before it actually hits the underlying disks.
Just was trying to determine a way to mirror this setup with XFS.
Thanks,
Ray
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