How much RAM do I need for ZFS cache?

Configuring Max Memory Limit for ARC On Linux, ZFS uses 50% of the installed memory for ARC caching by default. So, if you have 8 GB of memory installed on your computer, ZFS will use 4 GB of memory for ARC caching at max.

What is ZFS Zil?

ZIL stands for ZFS Intent Log. The purpose of the ZIL in ZFS is to log synchronous operations to disk before it is written to your array. That synchronous part essentially is how you can be sure that an operation is completed and the write is safe on persistent storage instead of cached in volatile memory.

How big should my L2ARC be?

“As a general rule of thumb, an L2ARC should not be added to a system with less than 64 GB of RAM and the size of an L2ARC should not exceed 5x the amount of RAM. In some cases, it may be more efficient to have two separate pools: one on SSDs for active data and another on hard drives for rarely used content.

Does ZFS use a lot of RAM?

Otherwise, it is basically the SLUB memory block allocator that was used in the Linux kernel for a while. So yes, it can run on watchOS-level amount of RAM. ZFS data deduplication does not require much more ram than the non-deduplicated case.

Does ZFS require more RAM?

ZFS on FreeNAS typically requires a base 8GB plus an additional 1GB per TB of disk space to get “decent” performance. This softens a bit as the number of TB managed gets out past maybe 20 or so. But some more demanding workloads will require substantially more RAM.

Does ZFS need cache?

ZFS allows for tiered caching of data through the use of memory. The first level of caching in ZFS is the Adaptive Replacement Cache (ARC), once all the space in the ARC is utilized, ZFS places the most recently and frequently used data into the Level 2 Adaptive Replacement Cache (L2ARC).

Should I use L2ARC?

When should I use L2ARC? For most users, the answer to this question is simple—you shouldn’t. The L2ARC needs system RAM to index it—which means that L2ARC comes at the expense of ARC.

What is ARC in ZFS?

The “ARC” is the ZFS main memory cache (in DRAM), which can be accessed with sub microsecond latency. An ARC read miss would normally read from disk, at millisecond latency (especially random reads).

Does ZFS write to ZiL log?

However for ZFS writes and cache flushes trigger ZIL event log entries. The end result is that the ZFS array will end up doing a massively disproportional amount of writing to the ZIL log and throughput will suffer (I was seeing under 1 MiB/sec on Gigabit Ethernet!).

How bad is ZFS performance on Gigabit Ethernet?

The end result is that the ZFS array will end up doing a massively disproportional amount of writing to the ZIL log and throughput will suffer (I was seeing under 1 MiB/sec on Gigabit Ethernet!). Here are the results of testing the various work-arounds, as you can see that modifying the kernel is the clear winner.

Why does fragmentation occur in Zil?

When the ZIL Transaction is commited to disk, the deletion of the gang block also implies the deletion of every gang members and its childs. This is where fragmentation occurs! Every ZIL transaction that is allocated and then freed on disk cause gap to appear between ZIL’s entries and pool’s data.

What’s wrong with ZFS-backed NFS for datastore?

There is a special issue when using ZFS-backed NFS for a Datastore under ESXi. The problem is that the ESXi NFS client forces a commit/cache flush after every write. This makes sense in the context of what ESXi does as it wants to be able to reliably inform the guest OS…