fail: doveadm backup -s "<state>" with a huuge number of mailboxes

[Heiko Schlittermann]

Timo Sirainen <tss at iki.fi> (Do 12 Nov 2015 23:12:21 CET):
…
> > The user has about 4501 IMAP folders. This makes a huge
> > state output of about 128 kB. This in turn leads to an overly long
> > command line in the next iteration. (excuse: command line too long)
> > 
> > Is there any chance to solve this? I think, the only possibility would
> > be to have doveadm-backup -s read the state from a *file*, wouldn't it?
> 
> Right now no way to read it from a file. But is this Linux? I thought Linux had unlimited (or at least very large) command line length nowadays.


Yes, the product of size and number of the arguments is limited:

    *** 189961 args (    10 chars each) with total length 1899610
    ***  99503 args (    20 chars each) with total length 1990060
    ***   2087 args (  1000 chars each) with total length 2087000
    ***     16 args (128000 chars each) with total length 2048000

The upper limit for the sum of parameters seems to be ~2 MB

But,

     x127=$(perl -e 'print "x" x (127 * 2**10)')    # 127 KB 
     x128=$(perl -e 'print "x" x (128 * 2**10)')    # 128 KB

     /bin/echo $x127                                # works
     /bin/echo $x127 $x127                          # works

     /bin/echo $x128                                # breaks
     /bin/echo $x128 $x128                          # breaks

It seems that a *single* command line parameter is limited to somewhat
near 128 kB. The complete amount of command line data is near 2 MB.

Conclusion: doveadm sync should be able to read a state *file*. IMHO

    Best regards from Dresden/Germany
    Viele Grüße aus Dresden
    Heiko Schlittermann
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