Explaining The DRBD® Configuration ‘al-extents.’

al-extents

There can sometimes be a lot of confusion and complexity about the DRBD® configuration value al-extents (activity log extents). We have written this blog post to shed some light on the subject. We hope you find it useful.

Imagine a node having a DRBD resource in the secondary role. When it crashes, gets powered down, or loses connection to the primary for any other reason, the primary will remember the changed blocks in its bitmap. So when the secondary node connects to the primary again, the primary can tell precisely what data has to be transferred to get it Up-to-Date again.

Now, when the node with the Primary role crashes, no one can say with certainty which blocks have been changed on its storage but hasn’t been sent to the secondary, or vice-versa.

The primary has to persistently remember which blocks it touched lately so that on a crash. It can ask the secondary node (which might have been promoted to primary in the meantime) for these blocks. They are fetched and written to the crashed primaries’ storage to get the data consistent with the other node.

The al-extents value defines how big the hot area (=active set) can get. As the AL has to be stored on persistent storage, each change to this set has to be written to the meta-data area before the actual data can be written. Waiting for that can slow down DRBD. So you can choose between:

  • A small activity log – this might make DRBD slower during load.
  • Or a big activity log will cause more data to be re-synced in case of a primary crash.

So, for the most part, people are happily choosing a bigger activity log; in the 8.3 series, the maximum value is 3833. For 8.4, the limit rose to 6433, and who knows what 9.0 will bring? Contact the team if you want to discuss anything within this article.

Yusuf Yıldız

Yusuf Yıldız

After nearly 15 years of system and storage management, Yusuf started to work as a solution architect at LINBIT. Yusuf's main focus is on customer success and contributing to product development and testing. As part of the solution architects team, he is one of the backbone and supporter of the sales team.

Talk to us

LINBIT is committed to protecting and respecting your privacy, and we’ll only use your personal information to administer your account and to provide the products and services you requested from us. From time to time, we would like to contact you about our products and services, as well as other content that may be of interest to you. If you consent to us contacting you for this purpose, please tick above to say how you would like us to contact you.

You can unsubscribe from these communications at any time. For more information on how to unsubscribe, our privacy practices, and how we are committed to protecting and respecting your privacy, please review our Privacy Policy.

By clicking submit below, you consent to allow LINBIT to store and process the personal information submitted above to provide you the content requested.

Talk to us

LINBIT is committed to protecting and respecting your privacy, and we’ll only use your personal information to administer your account and to provide the products and services you requested from us. From time to time, we would like to contact you about our products and services, as well as other content that may be of interest to you. If you consent to us contacting you for this purpose, please tick above to say how you would like us to contact you.

You can unsubscribe from these communications at any time. For more information on how to unsubscribe, our privacy practices, and how we are committed to protecting and respecting your privacy, please review our Privacy Policy.

By clicking submit below, you consent to allow LINBIT to store and process the personal information submitted above to provide you the content requested.