[***] indicates that text has been omitted which is the subject of a
confidential treatment request. This text has been filed separately with the
SEC.

                                                                   Exhibit 10.20


                                  ADDENDUM TWO
                                     TO THE
                       LICENSE AND DISTRIBUTION AGREEMENT
                                     BETWEEN
                               DELL PRODUCTS L.P.
                                       AND
                             COMMVAULT SYSTEMS, INC.

This Addendum Two ("Addendum Two") to the Software License Agreement dated
December 17, 2003 (the "Agreement"), is entered into by and between DELL
PRODUCTS L.P. (HEREINAFTER "DELL") WITH ITS PRINCIPAL PLACE OF BUSINESS AT ONE
DELL WAY, ROUND ROCK, TEXAS 78682, AND COMMVAULT SYSTEMS, INC., (HEREINAFTER
"COMMVAULT") A DELAWARE CORPORATION HAVING A PRINCIPAL PLACE OF BUSINESS AT 2
CRESCENT PLACE, OCEANPORT, NEW JERSEY 07757 AND IS EFFECTIVE AS OF THE 30TH DAY
OF OCTOBER, 2004 (THE "EFFECTIVE DATE").

                                    RECITALS

WHEREAS, COMMVAULT and Dell entered into the Agreement through which COMMVAULT
granted Dell various rights to distribute certain COMMVAULT software products;

WHEREAS, the parties now desire to amend the Agreement.

NOW THEREFORE, in consideration of the mutual covenants and promises set forth
herein and for other good and valuable consideration, the receipt of which both
parties hereby acknowledge, Dell and COMMVAULT agree as follows:

Any capitalized terms defined in this Addendum are specific to this Addendum
only, and do not modify or change the meaning set forth in the Agreement. Unless
expressly defined in this Addendum, the capitalized terms in this Addendum are
as defined in the Agreement. The Agreement shall remain in full force and effect
except as supplemented and amended herein.

      1.    The Pricing Supplement to the Agreement is hereby replaced in its
            entirety with the attached Exhibit B to this Addendum.

      2.    [***]. Dell shall coordinate with CommVault support to ensure that
            customers who receive support and product updates are eligible for
            such services. CommVault's MSRP for annual maintenance contracts is
            [***] of product list price. Dell's cost to CommVault is [***] of
            software cost and reflected in the Exhibit B to this Addendum.

No other changes or modification are intended by this Addendum. All other terms
and conditions of the Agreement are in effect.

[***] indicates that text has been omitted which is the subject of a
confidential treatment request. This text has been filed separately with the
SEC.

IN WITNESS WHEREOF, the parties have executed this Addendum by their duly
authorized representatives as of the date first set forth above.


COMMVAULT SYSTEMS INC.                      DELL PRODUCTS L.P.

By:       /s/ David R. West                 By:     /s/ Joseph Kanicki
   -----------------------------            -------------------------------

Name:  David R. West                        Name:  Joe Kanicki
     ---------------------------                 --------------------------

Title:   VP Business Development            Title:   Sr. Manager - WWSP
      --------------------------                  -------------------------

Date:   11/15/2004                          Date:   11/22/2004
     ---------------------------                 --------------------------



                                    EXHIBIT B

                               PRICING SUPPLEMENT
                        TO THE SOFTWARE LICENSE AGREEMENT
                           BETWEEN DELL PRODUCTS L.P.
                           AND COMMVAULT SYSTEMS INC.

<Table>
<Caption>
                                                                                                            DELL SW     1 YR. UPDATE
SKU    SKU DESCRIPTION                        [***]              [***]           MSRP        DISCOUNT         COST        PROTECTION
- ---    ---------------                        -----              -----           ----        --------         ----        ----------
                                                                                                   
                                                                 [***]
SKU1   CommServe                              [***]              [***]          $[***]        [***]          $[***]          $[***]
SKU2   [***]                                  [***]              [***]          $[***]        [***]          $[***]          $[***]
SKU3   [***]                                  [***]              [***]          $[***]        [***]          $[***]          $[***]
SKU4   [***]                                  [***]              [***]          $[***]        [***]          $[***]          $[***]
SKU5   [***]                                  [***]              [***]          $[***]        [***]          $[***]          $[***]
SKU6   [***]                                  [***]              [***]          $[***]        [***]          $[***]          $[***]
SKU7   [***]                                  [***]              [***]          $[***]        [***]          $[***]          $[***]
SKU8   [***]                                  [***]              [***]          $[***]        [***]          $[***]          $[***]
SKU9   [***]                                  [***]              [***]          $[***]        [***]          $[***]          $[***]
SKU10  [***]                                  [***]              [***]          $[***]        [***]          $[***]          $[***]
SKU11  [***]                                  [***]              [***]          $[***]        [***]          $[***]          $[***]
SKU12  [***]                                  [***]              [***]          $[***]        [***]          $[***]          $[***]
SKU13  [***]                                                     [***]          $[***]        [***]          $[***]          $[***]
SKU14  GalaxyExpress Server Suite
          includes:                           [***]              [***]          $[***]        [***]          $[***]          $[***]
          GalaxyExpress (key code
             upgrade capability to Galaxy)
          1 Windows or Storage Server
             2003 MediaAgent included
          Single tape drive support
             (no library support)
          5 FS Agents (can purchase more
             FS agents from list above) for
             Windows, UNIX, Linux or Novell
          May purchase Windows Application
             Agents from above
          Max 10 total clients (agents)

SKU15  GalaxyExpress Automation Suite
          includes:                           [***]              [***]          $[***]        [***]          $[***]          $[***]
          GalaxyExpress (key code upgrade
             capability to Galaxy)
          1 Windows, Storage Server 2003,
             Linux or Novell MediaAgent
             included
          Media Agent supports a maximum
             of one 6-drive library
          5 FS Agents (can purchase more
             FS agents from list above) for
             Windows, UNIX, Linux or Novell
          Max 10 total clients (agents)
             inclusive of coupon for 1
             Windows Application
             IDA
          (may purchase more Windows
             (only) application agents
             from list above)

SKU16  [***]                                                                    $[***]        [***]          $[***]          $[***]
SKU17  [***]                                                                    $[***]        [***]          $[***]          $[***]
SKU18  [***]                                                                    $[***]        [***]          $[***]          $[***]
SKU19  [***]                                                                    $[***]        [***]          $[***]          $[***]
</Table>