[Dovecot] index IO patterns

[Timo Sirainen]

On 11.5.2012, at 13.56, Javier de Miguel Rodríguez wrote:

>> Even without LDA/LMTP dovecot-imap needs to write right? It would
>> need to update the index every time an imap connect happens and
>> new mails are found in the mail store.
> 
> Well of course. Indexes are also updated when flags are modified, moved a messages, delete a message, etc.. But in my setup there are 65% reads and the rest writes


There are several hard coded values related to read/write percentages. If you're interested you could try if changing them increases the read%:

mail-index-private.h:

/* Write to main index file when bytes-to-be-read-from-log is between these
   values. */
#define MAIL_INDEX_MIN_WRITE_BYTES (1024*8)
#define MAIL_INDEX_MAX_WRITE_BYTES (1024*128)

mail-cache-private.h:

/* Never compress the file if it's smaller than this */
#define MAIL_CACHE_COMPRESS_MIN_SIZE (1024*50)

/* Compress the file when deleted space reaches n% of total size */
#define MAIL_CACHE_COMPRESS_PERCENTAGE 20

/* Compress the file when n% of rows contain continued rows.
   200% means that there's 2 continued rows per record. */
#define MAIL_CACHE_COMPRESS_CONTINUED_PERCENTAGE 200

Increasing this might also improve read performance, compat.h:

/* Try to keep IO operations at least this size */
#ifndef IO_BLOCK_SIZE
#  define IO_BLOCK_SIZE 8192
#endif

All of these are just runtime checks (not saved anywhere), so there's no danger in changing them.