doveadm backup mdbox - initial copy slow

[Gerald Galster]

> On 30.8.2019 12.33, Gerald Galster via dovecot wrote:
>> Hello,
>> 
>> when calling doveadm backup like in the following example, it seems to flush every email to disk
>> maxing out harddrives (iops) and resulting in very poor performance:
>> 
>> Calling doveadm -o plugin/quota= backup -u "statusmails at domain.com" "mdbox:/backup/vmail/domain.com/statusmails"
>> finished in 212 secs [changes:1800 copy:1800 delete:0 expunge:0]
>> 
>> The source mdbox holds 1800 small emails and uses about 20 MB disk space only.
>> 
>> I've tried -f (full sync) and -l 30 (locking the mailbox) which did not get faster, nor did doveadm sync -1.
>> 
>> When using doveadm import all mails are copied in less than 3 seconds:
>> time doveadm -o mail=mdbox:/backup/vmail/domain.com/statusmails import mdbox:/var/vmail/domain.com/statusmails "" all
>> real	0m2.605s
>> 
>> Are there any other options I could try to speed up doveadm backup, avoiding the flush after each email?
>> 
>> This is doveadm from dovecot 2.2 (>= 2.2.33). Does anyone get better results with 2.3 tree?
>> 
>> Best regards
>> Gerald
> 
> Try setting mail_fsync=never

Thanks Aki, that did the trick!

Calling /usr/bin/doveadm -o plugin/quota= -o mail_fsync=never backup -u "statusmails at domain.com" "mdbox:/backup/vmail/domain.com/statusmails"
finished in 2 secs [changes:1925 copy:1925 delete:0 expunge:0]

Best regards
Gerald