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Re: [PATCH v2] virtiofsd/passthrough_ll: don't remove O_DIRECT when cach
From: |
Stefan Hajnoczi |
Subject: |
Re: [PATCH v2] virtiofsd/passthrough_ll: don't remove O_DIRECT when cache=none |
Date: |
Thu, 30 Apr 2020 09:35:43 +0100 |
On Sat, Apr 11, 2020 at 04:35:44AM -0400, Catherine Ho wrote:
> @@ -1702,10 +1703,11 @@ static void update_open_flags(int writeback, struct
> fuse_file_info *fi)
>
> /*
> * O_DIRECT in guest should not necessarily mean bypassing page
> - * cache on host as well. If somebody needs that behavior, it
> - * probably should be a configuration knob in daemon.
> + * cache on host as well. If cache=none, keep the flag unchanged
> */
> - fi->flags &= ~O_DIRECT;
> + if (cache_mode != CACHE_NONE) {
> + fi->flags &= ~O_DIRECT;
> + }
> }
>
Thanks for the patch! I have CCed the virtio-fs mailing list so more
people see it.
Please add a new command-line option to control O_DIRECT behavior.
There are two cases:
1. O_DIRECT bypasses the guest page cache but not the host page cache.
This makes sense when the DAX feature is enabled.
2. O_DIRECT bypasses both the guest and host page cache. This make
sense for non-DAX and for I/O performance benchmarking.
Today only #1 is supported.
Your patch makes the behavior dependent on the cache mode option, but
the cache mode doesn't necessarily determine how O_DIRECT should be
handled. For example, in the DAX case the guest page cache is bypassed
and cache=none can be used, but we do want to use the host page cache.
You can add a new option so that O_DIRECT handling is configurable for
all cases.
Stefan
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