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@ -1726,18 +1726,41 @@ unused files to be released from memory. |
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.PP |
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EXPERIMENTAL |
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.PP |
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This preloadable library overrides the creation and opening of files in |
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order to simulate passthrough file IO. |
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It catches the open/creat/fopen calls, lets mergerfs do the call, |
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queries mergerfs for the branch the file exists on, and reopens the file |
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on the underlying filesystem. |
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Meaning that you will get native read/write performance. |
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.PP |
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This will only work on dynamically linked software. |
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For some time there has been work to enable passthrough IO in FUSE. |
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Passthrough IO would allow for near native performance with regards to |
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reads and writes (at the expense of certain mergerfs features.) However, |
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there have been several complications which have kept the feature from |
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making it into the mainline Linux kernel. |
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Until that feature is available there are two methods to provide similar |
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functionality. |
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One method is using the LD_PRELOAD feature of the dynamic linker. |
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The other leveraging ptrace to intercept syscalls. |
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Each has their disadvantages. |
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At the moment only a preload based tool is available. |
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A ptrace based tool may be developed later if there is a need. |
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.PP |
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\f[C]/usr/lib/mergerfs/preload.so\f[R] |
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.PP |
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This preloadable |
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library (https://man7.org/linux/man-pages/man8/ld.so.8.html#ENVIRONMENT) |
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overrides the creation and opening of files in order to simulate |
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passthrough file IO. |
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It catches the open/creat/fopen calls, has mergerfs do the call, queries |
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mergerfs for the branch the file exists on, reopens the file on the |
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underlying filesystem and returns that instead. |
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Meaning that you will get native read/write performance because mergerfs |
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is no longer part of the workflow. |
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Keep in mind that this also means certain mergerfs features that work by |
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interrupting the read/write workflow, such as \f[C]moveonenospc\f[R], |
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will no longer work. |
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.PP |
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Also understand that this will only work on dynamically linked software. |
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Anything statically compiled will not work. |
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Many GoLang and Rust apps are statically compiled. |
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.PP |
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The library will not interfere with non-mergerfs filesystems. |
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The library is written to always fallback to returning the mergerfs |
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opened file on error. |
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.PP |
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While the library was written to account for a number of edgecases there |
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could be some yet accounted for so please report any oddities. |
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@ -1754,22 +1777,52 @@ LD_PRELOAD=/usr/lib/mergerfs/preload.so touch /mnt/mergerfs/filename |
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.SS Docker usage |
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.PP |
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Assume \f[C]/mnt/fs0\f[R] and \f[C]/mnt/fs1\f[R] are pooled with |
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mergerfs at \f[C]/mnt/mergerfs\f[R]. |
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mergerfs at \f[C]/media\f[R]. |
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.PP |
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Remember that you must bind into the container the original host paths |
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to the same locations otherwise the preload module will not be able to |
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find the files. |
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All mergerfs branch paths \f[I]must\f[R] be bind mounted into the |
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container at the same path as found on the host so the preload library |
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can see them. |
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.IP |
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.nf |
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\f[C] |
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docker run \[rs] |
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-e LD_PRELOAD=/usr/lib/mergerfs/preload.so \[rs] |
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-v /usr/lib/mergerfs/preload.so:/usr/lib/mergerfs/preload.so:ro \[rs] |
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-v /media:/data \[rs] |
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-v /mnt:/mnt \[rs] |
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ubuntu:latest \[rs] |
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bash |
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\f[R] |
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.fi |
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.PP |
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or more explicitly |
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.IP |
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.nf |
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\f[C] |
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docker run \[rs] |
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-e LD_PRELOAD=/usr/lib/mergerfs/preload.so \[rs] |
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-v /usr/lib/mergerfs/preload.so:/usr/lib/mergerfs/preload.so:ro \[rs] |
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-v /media:/data \[rs] |
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-v /mnt/fs0:/mnt/fs0 \[rs] |
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-v /mnt/fs1:/mnt/fs1 \[rs] |
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ubuntu:latest \[rs] |
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bash |
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\f[R] |
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.fi |
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.SS systemd unit |
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.PP |
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Use the \f[C]Environment\f[R] option to set the LD_PRELOAD variable. |
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.IP \[bu] 2 |
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https://www.freedesktop.org/software/systemd/man/latest/systemd.service.html#Command%20lines |
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.IP \[bu] 2 |
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https://serverfault.com/questions/413397/how-to-set-environment-variable-in-systemd-service |
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.IP |
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.nf |
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\f[C] |
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[Service] |
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Environment=LD_PRELOAD=/usr/lib/mergerfs/preload.so |
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\f[R] |
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.fi |
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.SS Misc |
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.IP \[bu] 2 |
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https://github.com/trapexit/mergerfs-tools |
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trapexit
11 months ago
GitHub