Primocache Questions

[mell111]

Hi,

I have a few questions about primocache:

1. When using it in write (only) mode with an ssd acting as L2 cache for a hard drive - will it issue a trim command to the ssd when the data is sync'd to the hard drive? If so, how often? If not, is there a way to clear the cache and issue a manual trim (with Windows built in Optimize Drive)?

2. With lots of RAM and also in write only mode, can it be configured to only write to the ssd when the RAM cache fills up? Does the cache buffer get invalidated automatically as data is sync'd to the hard drive?

3. Can multiple ssd's be used as L2 cache?

4. With a clean Windows shutdown, will it automatically detect and sync all data, or is a manual sync required?

5. In case of a system crash, how does running with primocache compare with regular Windows (non-cached) as far as NTFS volume corruption (aside from obviously losing unsync'd cache data and possible open file corruption). From my experience, NTFS is quite resilient in recovering from a system crash and I'm just wondering how primocache affects that?

Thanks in advance!

[Support]

1. When using it in write (only) mode with an ssd acting as L2 cache for a hard drive - will it issue a trim command to the ssd when the data is sync'd to the hard drive? If so, how often? If not, is there a way to clear the cache and issue a manual trim (with Windows built in Optimize Drive)?

So far no. But we'll support and try to issue trim commands to L2 cache.

2. With lots of RAM and also in write only mode, can it be configured to only write to the ssd when the RAM cache fills up? Does the cache buffer get invalidated automatically as data is sync'd to the hard drive?

So far no. PrimoCache will try to flush some deferred write data in L1 (memory) cache to disk, in order to get best write performance.
If the option "Free Cache on Written" is not checked, write-data is still valid after data is sync'd to the drive. This is to get better read performance in case that these write-data will be read-back.

3. Can multiple ssd's be used as L2 cache?

No. But you can use a soft-raid or hardware-raid to merge multiple SSD's into one volume.

4. With a clean Windows shutdown, will it automatically detect and sync all data, or is a manual sync required?

Yes, PrimoCache automatically flushes all "dirty" write-data to the drive at a clean shutdown. No manual sync is required.

5. In case of a system crash, how does running with primocache compare with regular Windows (non-cached) as far as NTFS volume corruption (aside from obviously losing unsync'd cache data and possible open file corruption). From my experience, NTFS is quite resilient in recovering from a system crash and I'm just wondering how primocache affects that?

Sorry, the result is unexpected. PrimoCache has no chance to flush "dirty" write-data to target drives at a Windows crash. All these dirty data will be lost and this might corrupt volume file system.

[mell111]

Thanks for the reply.

To clarify, when you say:

"So far no. But we'll support and try to issue trim commands to L2 cache."

Do you mean in a future version? Do you think it will be in the near future? (I know you can't promise anything, I'm just wondering if it's considered a priority.) SSDs today are typically TLC which have only about 1000 write cycles. With write amplification that can go fast. Do you have any information on SSD write amplification when PrimoCache is used with an SSD as L2 cache?

Regarding item 2., I don't think I was clear enough: I'm wondering with an L1 (RAM) and L2 (SSD) setup if the L1 cache will always be flushed to L2 before going to disk, or if it will go straight to disk, bypassing the L2 SSD (to spare writes to the SSD.) Is the L2 cache only overflow for the L1? This would be with appropriate settings in PrimoCache, of course.

Thanks again.

[Support]

Do you mean in a future version? Do you think it will be in the near future?

We give this task normal priority. Recently we still have some higher priority tasks to do. But we'll support this task as soon as possible.

I'm wondering with an L1 (RAM) and L2 (SSD) setup if the L1 cache will always be flushed to L2 before going to disk, or if it will go straight to disk, bypassing the L2 SSD (to spare writes to the SSD.) Is the L2 cache only overflow for the L1?

In current design, L1 is always be flushed to disk directly.