[Vdo-devel] Questions on write policy, discard and thin provisioning

[Gionatan Danti]

Hi all,
after reading RedHat documentation [1], I have some question about VDO.

1) On the linked documentation I read: "When VDO is in sync mode, writes 
to the VDO device are acknowledged when the underlying storage has 
written the data permanently. When VDO is in async mode, writes are 
acknowledged before being written to persistent storage"

Am I right reading it as "in sync mode, writes to the VDO device are 
acknowledged when the underlying storage *is expected to* have written 
the data permanently"? In other words: sync mode disable upper layer 
FLUSHes/FUAs, while async mode advertise the volatile cache so that 
upper layers can flush at appropriate time?

What about using async with a non-volatile write cache (ie: BBU RAID 
cards)? As barriers will be ignored by such devices, I expect a minor 
performance impact only, right?

2) on this interesting reply [2] I read that "VDO does not send discards 
to the storage device beneath it". Does it means that flash-based 
storage can not have any benefit from upper layer trims?

3) when using VDO for virtual machine storage the recommended ratio is 
10:1 - ie advertise a 10 TB device with a real 1 TB disk. Great. But 
what happen if the compression/deduplication ratio is below expectation, 
maybe way below the 10:1 ratio? I expect the VDO to fill, with the upper 
layer filesystem ignoring the problem (ie: it see much "free space"). 
With lvm thin provisining this is a serious problem which was thoroughly 
discussed in the mailing list. What about VDO?

4) does VDO provide some kind of snapshot, or should I continue to use 
lvm thin snapshot (on top of a VDO device) for this purpose?

Thanks.


[1] 
https://access.redhat.com/documentation/en-us/red_hat_enterprise_linux/7/html/storage_administration_guide/vdo-qs-creating-a-volume

[2] https://www.redhat.com/archives/vdo-devel/2018-February/msg00009.html

-- 
Danti Gionatan
Supporto Tecnico
Assyoma S.r.l. - www.assyoma.it
email: g.danti at assyoma.it - info at assyoma.it
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