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Re: A blocksize problem about dax and ext4

from [Cholerae Hu] [Permanent Link][Original]
To: "Elliott, Robert (Persistent Memory)" <elliott@xxxxxxx>
Subject: Re: A blocksize problem about dax and ext4
From: Cholerae Hu <choleraehyq@xxxxxxxxx>
Date: Thu, 24 Dec 2015 12:13:26 +0800
Cc: Dan Williams <dan.j.williams@xxxxxxxxx>, Dave Chinner <david@xxxxxxxxxxxxx>, Ted Tso <tytso@xxxxxxx>, "adilger.kernel@xxxxxxxxx" <adilger.kernel@xxxxxxxxx>, "linux-nvdimm@xxxxxxxxxxxx" <linux-nvdimm@xxxxxxxxxxxx>, "linux-ext4@xxxxxxxxxxxxxxx" <linux-ext4@xxxxxxxxxxxxxxx>, "xfs@xxxxxxxxxxx" <xfs@xxxxxxxxxxx>, "linux-kernel@xxxxxxxxxxxxxxx" <linux-kernel@xxxxxxxxxxxxxxx>
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I used ext4 and set the blocksize to 4096, the mount command was silent. Does it indicate that dax has already been enabled?

2015-12-24 10:47 GMT+08:00 Elliott, Robert (Persistent Memory) <elliott@xxxxxxx>:
> -----Original Message-----
> From: Cholerae Hu [mailto:choleraehyq@xxxxxxxxx]
> Sent: Wednesday, December 23, 2015 8:36 PM
> Subject: Re: A blocksize problem about dax and ext4
...
> xfs will silently disable dax when the fs block size is too small,
> i.e. your mmap() operations are backed by page cache in this case.
> Currently the only indication of whether a mapping is DAX backed or
> not is the presence of the VM_MIXEDMAP flag ("mm" in the VmFlags field
> of /proc/<pid>/smaps)
>
> Did you mean that I should make the blocksize bigger until the mount
> command tell me that dax is enabled?

To really use DAX, the filesystem block size must match the
system CPU's page size, which is probably 4096 bytes.

---
Robert Elliott, HPE Persistent Memory


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