SSD TRIM Support and Block Limits?

Question

I'm running Ubuntu 13.10. In this situation, I have two questions?

1. I have 4 drives on my desktop, two of which are SSD's with TRIM supported. However the two have different block limits. I am trying to understand the difference in this:

   Model Number:       Samsung SSD 840 PRO Series              
      *    Data Set Management TRIM supported (limit 8 blocks)
   
   Model Number:       SanDisk SDSSDX120GG25                   
      *    Data Set Management TRIM supported (limit 1 block)
   

   So, what is the difference between limit 8 blocks and limit 1 block? Which would be better?

2. I've read articles/tips on enabling trip for previous versions of Ubuntu, how can check to see if I currently have TRIM enabled?

Thank you for your time.

Answer 1

1. The N in "limit N blocks" is the maximum number of 512-byte blocks of LBA Range Entries your SSD can handle in a single DATA SET MANAGEMENT command. So we could say that a higher N is better, but according to wikipedia:

   Typically this defaults to 8 (or 4 kB) but many drives reduce this to one to meet the Microsoft Windows Hardware Requirements for Trim, Command completion time shall not exceed 20 ms or 8 ms * (number of LBA range entries), whichever is greater, and shall always be less than 600 ms.

   So for me it is not obvious that every drive wants to comply with Microsoft's requirement. If we assume that both of your SSD's comply with those requirements we can say that your Samsung SSD is better in this subject, since it can TRIM more blocks in the same time than your Sandisk one.

   Your Samsung SSD can TRIM a maximum of 16GB of data in a single TRIM command, your Sandisk SSD can only TRIM a maximum of 2 GB of data in a single TRIM command. (That maximum is only reachable if the LBAs to TRIM are contiguous.) But as I said before, we don't know that how fast each of these SSD's deal with that single TRIM command, if they need the same time, then obviously Samsung is better, if the Sandisk finishes much faster with that single command, it may be able to run more TRIM commands in the same time the Samsung runs only one TRIM command.

   Anyway, I think that this parameter has no importance for a consumer. This is more like just a technical characteristic of the drive.

2. If you mount your SSD with the discard flag, or if you use fstrim (maybe in a cron job) then you have TRIM enabled. There is nothing more to do to enable it.