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# Allow all users to mount a USB drive.
own /dev/da0 root:operator
perm /dev/da0 0666
All users can now mount devices they could read onto a directory that they own:
<prompt>%</prompt> <userinput>mkdir ~/my-mount-point</userinput>
<prompt>%</prompt> <userinput>mount -t msdosfs /dev/da0 ~/my-mount-point</userinput>
Unmounting the device is simple:
<prompt>%</prompt> <userinput>umount ~/my-mount-point</userinput>
Enabling <varname>vfs.usermount</varname>, however, has negative security implications. A better way to access <trademark class="registered">MS-DOS</trademark> formatted media is to use the <package>emulators/mtools</package> package in the Ports Collection.
The device name used in the previous examples must be changed according to the configuration.
The <command>du</command> and <command>df</command> commands show different amounts of disk space available. What is going on?
This is due to how these commands actually work. <command>du</command> goes through the directory tree, measures how large each file is, and presents the totals. <command>df</command> just asks the file system how much space it has left. They seem to be the same thing, but a file without a directory entry will affect <command>df</command> but not <command>du</command>.
When a program is using a file, and the file is deleted, the file is not really removed from the file system until the program stops using it. The file is immediately deleted from the directory listing, however. As an example, consider a file large enough to affect the output of <command>du</command> and <command>df</command>. A file being viewed with <command>more</command> can be deleted wihout causing an error. The entry is removed from the directory so no other program or user can access it. However, <command>du</command> shows that it is gone as it has walked the directory tree and the file is not listed. <command>df</command> shows that it is still there, as the file system knows that <command>more</command> is still using that space. Once the <command>more</command> session ends, <command>du</command> and <command>df</command> will agree.
This situation is common on web servers. Many people set up a FreeBSD web server and forget to rotate the log files. The access log fills up <filename>/var</filename>. The new administrator deletes the file, but the system still complains that the partition is full. Stopping and restarting the web server program would free the file, allowing the system to release the disk space. To prevent this from happening, set up <citerefentry><refentrytitle>newsyslog</refentrytitle><manvolnum>8</manvolnum></citerefentry>.
Note that Soft Updates can delay the freeing of disk space and it can take up to 30 seconds for the change to be visible.
How can I add more swap space?
This section <link xlink:href="@@URL_RELPREFIX@@/doc/en_US.ISO8859-1/books/handbook/adding-swap-space.html">of the Handbook</link> describes how to do this.
Why does FreeBSD see my disk as smaller than the manufacturer says it is?
Disk manufacturers calculate gigabytes as a billion bytes each, whereas FreeBSD calculates them as 1,073,741,824 bytes each. This explains why, for example, FreeBSD's boot messages will report a disk that supposedly has 80 GB as holding 76,319 MB.
Also note that FreeBSD will (by default) <link linkend="disk-more-than-full">reserve</link> 8% of the disk space.
How is it possible for a partition to be more than 100% full?
A portion of each UFS partition (8%, by default) is reserved for use by the operating system and the <systemitem class="username">root</systemitem> user. <citerefentry><refentrytitle>df</refentrytitle><manvolnum>1</manvolnum></citerefentry> does not count that space when calculating the <literal>Capacity</literal> column, so it can exceed 100%. Notice that the <literal>Blocks</literal> column is always greater than the sum of the <literal>Used</literal> and <literal>Avail</literal> columns, usually by a factor of 8%.
For more details, look up <option>-m</option> in <citerefentry><refentrytitle>tunefs</refentrytitle><manvolnum>8</manvolnum></citerefentry>.
What is the minimum amount of RAM one should have to run ZFS?
A minimum of 4GB of RAM is required for comfortable usage, but individual workloads can vary widely.
What is the ZIL and when does it get used?
The <acronym>ZIL</acronym> (<acronym>ZFS</acronym> intent log) is a write log used to implement posix write commitment semantics across crashes. Normally writes are bundled up into transaction groups and written to disk when filled (<quote>Transaction Group Commit</quote>). However syscalls like <citerefentry><refentrytitle>fsync</refentrytitle><manvolnum>2</manvolnum></citerefentry> require a commitment that the data is written to stable storage before returning. The ZIL is needed for writes that have been acknowledged as written but which are not yet on disk as part of a transaction. The transaction groups are timestamped. In the event of a crash the last valid timestamp is found and missing data is merged in from the ZIL.
Do I need a SSD for ZIL?
By default, ZFS stores the ZIL in the pool with all the data. If an application has a heavy write load, storing the ZIL in a separate device that has very fast synchronous, sequential write performance can improve overall system performance. For other workloads, a SSD is unlikely to make much of an improvement.
What is the L2ARC?
The <acronym>L2ARC</acronym> is a read cache stored on a fast device such as an <acronym>SSD</acronym>. This cache is not persistent across reboots. Note that RAM is used as the first layer of cache and the L2ARC is only needed if there is insufficient RAM.
L2ARC needs space in the ARC to index it. So, perversely, a working set that fits perfectly in the ARC will not fit perfectly any more if a L2ARC is used because part of the ARC is holding the L2ARC index, pushing part of the working set into the L2ARC which is slower than RAM.
Is enabling deduplication advisable?
Generally speaking, no.


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