most recent 30 from unix.stackexchange.com 2025-08-07T08:15:52Z https://unix.stackexchange.com/feeds/question/798475 https://creativecommons.org/licenses/by-sa/4.0/rdf https://unix.stackexchange.com/q/798475 8 adrifromhh https://unix.stackexchange.com/users/140346 2025-08-07T08:04:51Z 2025-08-07T23:15:23Z

<p>From my understanding, accessed files from disk are being kept in the page cache, for as long as there's enough memory left. Is this data also being moved to swap memory, or can this be facilitated?</p> <p><strong>I have the following case:</strong> A Debian 12 VPS server with limited (4GB) RAM and fairly large SSD storage. It is accessing files on a mounted SMB share, which is located on another server, over the internet through wireguard (bottleneck).</p> <p>Can I configure my VPS server so, that it caches as many of the files on the SMB share in local swap memory on it's SSD as possible? Or are there other tools to achieve this?</p> <p><strong>edit</strong> grawity's solution worked flawlessly!</p> <ol> <li><code>apt install cachefilesd</code></li> <li>Set <code>RUN=yes</code> in <code>/etc/default/cachefilesd</code></li> <li>Optionally set cache dir in <code>/etc/cachefilesd.conf</code></li> <li>Restart daemon <code>sudo systemctl restart cachefilesd.service</code></li> <li>Mount SMB share with <code>-o fsc</code> option</li> <li>Monitor <code>watch cat /proc/fs/fscache/stats</code></li> </ol>

https://unix.stackexchange.com/questions/798475/-/798478#798478 12 grawity https://unix.stackexchange.com/users/2168 2025-08-07T10:07:55Z 2025-08-07T10:07:55Z

<p>I don't know for sure, but I <em>really doubt</em> that such behavior would make sense generally, since the main purpose of a page cache is to avoid having to page-in stuff from disk, and the swap memory is... on disk... so if the page cache were swapped out, then the cost would be &quot;approximately the same&quot; to read it from swap versus reading it from the original disk. (As far as my understanding goes, it's not really designed for modern systems with different tiers of storage performance.)</p> <p>But there are other mechanisms for locally caching network filesystems, specifically the <em>fscache</em> framework:</p> <ul> <li><a href="https://docs.kernel.org/filesystems/caching/fscache.html" rel="noreferrer">https://docs.kernel.org/filesystems/caching/fscache.html</a></li> <li><a href="https://docs.kernel.org/filesystems/caching/cachefiles.html" rel="noreferrer">https://docs.kernel.org/filesystems/caching/cachefiles.html</a></li> <li><a href="https://docs.redhat.com/en/documentation/red_hat_enterprise_linux/6/html/storage_administration_guide/fscachesetup" rel="noreferrer">https://docs.redhat.com/en/documentation/red_hat_enterprise_linux/6/html/storage_administration_guide/fscachesetup</a></li> </ul> <p>Install the <code>cachefilesd</code> daemon, then mount your SMB filesystem with the <code>fsc</code> option:</p> <pre><code>mount -t smb3 //foo/bar /mnt/foo -o fsc </code></pre> <p>If it works, the daemon will cache files to /var. I can't guarantee that it will work; cachefilesd has not seen much maintenance recently (and I believe was in the middle of a large rewrite).</p>

https://unix.stackexchange.com/questions/798475/-/798487#798487 10 AlexD https://unix.stackexchange.com/users/1027 2025-08-07T19:53:25Z 2025-08-07T19:53:25Z

<p>Only anonymous memory can be swapped out. By <a href="https://docs.kernel.org/admin-guide/mm/concepts.html#id6" rel="noreferrer">definition</a>, anonymous memory is memory that is not backed up by a filesystem.</p> <p>Page cache is backed by a filesystem, and under memory pressure, such pages are either discarded (if they haven't been modified) or written back directly to the backing filesystem.</p> <p>See <a href="https://docs.kernel.org/admin-guide/mm/concepts.html" rel="noreferrer">Linux Kernel memory management documentation</a> for more details.</p>

https://unix.stackexchange.com/questions/798475/-/798492#798492 5 Austin Hemmelgarn https://unix.stackexchange.com/users/240409 2025-08-07T01:53:10Z 2025-08-07T01:53:10Z

<p>No, swap space is not used as backing for pages in the page cache (with one exception I will cover below), and it would be rather silly in most cases to do this, because swap space is generally expected to be no faster than the filesystem itself (and, in fact, it’s often a bit slower for multiple reasons).</p> <p>You’ve hit on one of the two cases where it theoretically would make sense to do this, namely using it as further cache for network filesystems, which are usually expected to be slower than local storage and thus slower than swap space. There’s pretty limited demand for that though given that normally you either don’t care about high performance for files on the network, or if you do you probably have the budget to throw lots of RAM at the problem, which will pretty much always be better for this than trying to use local persistent storage.</p> <p>The other case, which is the only situation in which pages from the page cache actually get pushed to swap space, is tmpfs. Normally on Linux, files in tmpfs exist only in the page cache (there is no persistent storage backing them). However, that obviously limits you to physical RAM capacity in terms of what you can store there, so there is special logic to allow pages that are part of files in a tmpfs instance to be moved from the page cache to swap space.</p> <blockquote> <p>Or are there other tools to achieve this?</p> </blockquote> <p>Given your stated goal of caching files from a network filesystem locally, what you want is this: <a href="https://docs.kernel.org/filesystems/caching/cachefiles.html" rel="noreferrer">https://docs.kernel.org/filesystems/caching/cachefiles.html</a></p> <p>In short, it uses a (presumably fast) local filesystem as a cache for files on a network filesystem. There’s a userspace component called ‘cachefilesd’ that’s used to manage it, usually installed in a package of the same name.</p> <p>You should definitely benchmark this though, given my own experience it’s only a net benefit for some workloads, not all.</p>

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