KVS - Limitations and behaviors of Keys

[keithpine]

I have some basic questions regarding the Motr Key-Value Store, particularly in regards to the keys:

1. Is there a maximum length imposed on either the key or value?
2. Does the key length affect the performance of KVS operations (lookup/insert/delete) at all?
3. Is there a recommended key size (e.g. <= 64 bytes), to optimize performance and storage?
4. What is the sorting/comparison order of the keys? My understanding is that, for something like a string key, it's a lexicographical order. Is that just done with unsigned byte comparisons (e.g. memcmp)?
5. Is it possible for a client to define a custom key comparison? For example, if I was using strings as keys, could I define a sorting function to store the keys in reverse order, so that NEXT key operations would return the keys in that reverse order? I see some ability to define this in Motr (m0_be_btree_kv_ops.ko_compare), but I assume that's internal and not available at the application layer.

[andriytk]

Hi @keithpine, let me try to answer. cc @nikitadanilov, @madhavemuri, @just-now, @ivan-alekhin, @huanghua78.

1. I think, the limit is imposed by the BE (back-end) maximum transaction size for key+value at the server side, which is configured with these parameters:

   cortx-motr/scripts/install/etc/sysconfig/motr

   Lines 87 to 88 in 47e7b4a

   	#MOTR_M0D_BETXGR_REG_SIZE_MAX=134217728
   	#MOTR_M0D_BETXGR_PAYLOAD_SIZE_MAX=134217728

2. Yes, limited by memcmp(k1, k2, len) performance on log(n) records in the index.
3. Not that I'm aware of.
4. See here:

   cortx-motr/motr/client.h

   Lines 369 to 372 in 47e7b4a

   	* An index stores records, each record consisting of a key and a value. Keys
   	* and values within the same index can be of variable size. Keys are ordered by
   	* the lexicographic ordering of their bit-level representation. Records are
   	* ordered by the key ordering. Keys are unique within an index.

5. No, I'm afraid such capability is not exposed on the Motr API level.