[SOLVED] VM disk cache option with ISCSI (SAN)

[Alex24]

Hello,

We use Proxmox 7 cluster, VM storage is directly connected with 2 SAN (Dell PowerVault MD), storage type in proxmox : LVM.


In the VM hardware -> Disk, the cache is set to "default (no cache).


With this type of storage hardwares (SAN), Is it better to keep with "default (no cache)" or to enable "write back" please ?
No risk to switch on an existing VM ?
Any other risk (more CPU, RAM...) ?

Thanks for your help.

 

[Hannes Laimer]

Hey,

the choice of disk caching policy can significantly affect the performance and safety of your data. Here's an overview of your options:

1. "No cache" mode: This is the safest mode. Data is written directly and synchronously to the disk. This can provide the lowest latency for clients, but at the cost of overall throughput.
2. "Write-back" mode: In this mode, Proxmox writes data into the cache first and then writes the data into the disk asynchronously. This provides high I/O performance and can be beneficial if you have applications with heavy write operations. However, it's also riskier because data could be lost if there's a power outage or system failure before the cached data is written to disk.

Switching from "no cache" to "write back" on an existing VM is generally safe, assuming your underlying storage subsystem (SAN in your case) can tolerate the potential data loss risk of write-back caching. However, note that enabling write-back caching can increase CPU and RAM usage, as it requires more system resources to manage the cache.

As for your SAN storage, it likely already has a built-in cache mechanism. Most SANs use a form of write-back caching internally. If this is the case, enabling "write back" at the Proxmox level might not give you a noticeable performance boost, because the write operations are already being cached at the SAN level.

In conclusion, if data integrity is the most critical aspect for your use case, it would be better to stick with "default (no cache)" mode. However, if you're willing to accept a slight risk of data loss for improved write performance, "write back" could be worth considering. Always ensure you have a robust backup system in place before enabling "write back", and monitor your system resources to ensure they aren't being overtaxed.

 

[Alex24]

Hello,

Thank you for your complete answer, It was exactly what I need to know for my case !

 

[nikybiasion]

With san, we have found that Write Throught has the best performance and is it the safest cache mode.

 

[Alex24]

Thanks for the feedback, we will take a try