From: "Elliott, Robert (Persistent Memory)" <elliott@hpe.com>
Subject: RE: A blocksize problem about dax and ext4
Date: Thu, 24 Dec 2015 02:47:07 +0000
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Cc: Ted Tso <tytso@mit.edu>,
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To: Cholerae Hu <choleraehyq@gmail.com>, Dan Williams
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> -----Original Message-----
> From: Cholerae Hu [mailto:choleraehyq@gmail.com]
> 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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